Cobb mentions in one scene that extradition between America and France is a bureaucratic nightmare. In Catch Me If You Can, one of Leo's previous roles, he played an American conman who was extradited from a French jail.
When the team realizes they need control of the first-class section of a plane in order to pull the heist, Saito announces that he's bought the airline. In The Aviator, when DiCaprio's character wanted to get into aviation professionally, he did the same.
Marion Cotillard won an Oscar for La Vie en Rose, a biopic of Edith Piaf. Piaf's song "Non je ne regrette rien" plays throughout the film; this allusion was non-intentional, the song was always to be in the film (Hans Zimmer wove it deeply into the score) and Cotillard's eventual casting just made it an odd connection.
Auteur License: Nolan spent years building up his reputation such directing mega hits like The Dark Knight to give him the clout to create this film his way. The fact that this film became a big hit itself guarantees he'll keep that clout for some time.
An inadvertent one. As mentioned above, Nolan cast Marion Cotillard, who played famous French singer Edith Piaf in the biopic La Vie en Rose, in a film that heavily features Piaf's song Je ne regrette rien. Christopher Nolan considered taking the song out of the movie when he cast Cotillard, but ultimately decided against after Hans Zimmer insisted — he had already built quite a bit of score around it.
It has been acknowledged that Nolan does seem to enjoy putting a bag over Murphy's head in his films.
Completely Different Title: While in the movie "inception" is used to describe an action ("incept an idea") many countries translated the title in the noun definition atop this page, mostly as "(The) Origin". Then there's China, where it's "Realm of the Dream Thieves" or "Comprehensive Launch".
This film was Nolan's baby, his decade-long dream project. You can see how much he loved what could have been an action-heist film, and how much enthusiasm the cast and crew had for him and the story. It's a real life heartwarmer, showing that there is a place for New Hollywood-style, auteur-driven projects. That said, making an art film after making hundreds of millions of dollars for your backers with The Dark Knight is more likely to work than coming in off the street. In fact, Inception is the reason Nolan took on The Dark Knight Saga to begin with. When working on the drafts for Inception, he realized he could not do it on a small-scale budget, and needed to get some experience with large-scale films, as well as every mainstream film he did that succeeded financially would make it that much easier for a studio to back whatever budget and resources he would need.
To add to that, Nolan tried as far as possible to film the spectacular effects in-camera, opting to use CGI to edit and enhance as opposed to entirely create. Massive props have to go to Chris Corbould, the special effects supervisor on the film who had to figure out how to achieve all the things (that had such a high Holy Shit Quotient) with practical effects. For example, the scene in the hotel bar with the slanting water levels was achieved with a hydraulically-controlled set that could tilt at a 25-degree angle, while the camera was anchored to the ground. Similarly, the zero-gravity fight in the hotel corridor was achieved with a rotating set, while the camera was kept level relative to the set. The effect was so amazing that the film's editor, Lee Smith, admits to being stunned and disoriented the first time he saw the footage. Also, the snow-fortress-hospital was built full-scale in the Canadian Rockies and blown up for real - along with a smaller scale model built in the studio parking lot. The atomizing streets of Paris were done with air cannons and rigs on an actual Paris street and the water flooding Saito's castle at the start of the film was accomplished with air cannons creating high-pressure jets of water.
The Penrose steps Arthur shows Ariadne? No CG trickery, they built the staircase then used the precise scaling angle for the camera to show off the optical illusion.
Also, he refused the studio's wish to have the movie converted to 3D as he isn't satisfied with the image quality of 3D.
Fake Nationality: Cillian Murphy, an Irishman, playing the Australian (judging by his passport) or American (judging by his accent; he could well be of mixed heritage) Robert Fischer. Presumably this applies to English actor Pete Postlethwaite (who plays Fischer's father) and Ellen Page as well, but their characters don't have their nationalities stated outright. Another possible example is Indian American Dileep Rao, who plays a vaguely Arab character called Yusuf.
The two female leads were up against each other for the Academy Award for "Outstanding Lead Actress in a Motion Picture" in the same year: Ellen Page for Juno and Marion Cotillard for La Vie en Rose. Cotillard won, but Page got her revenge. In Inception, her character shoots Marion's.
Arthur is played by Joseph Gordon-Levitt, who played Tommy on 3rd Rock from the Sun. The most memorable episodes of that show are the 3D episodes, which center on dreams. Tommy's own dream included him chasing a book down a hallway, that was not following the laws of physics. Also, he was a young man again, as he was actually the oldest of the aliens on his home planet.
Jossed: If Sir Michael Caine is to be believed, any fan theory which assumes that the "real world" is just another level of dream. Caine states that any scene which he's in is the real world. Virtually confirmed by Word of God, Christopher Nolan.
Magnum Opus Dissonance: This film was Nolan's dream project and it did pay off and became highly successful and appreciated...it's just that The Dark Knight Saga still overshadows it when it comes to Nolan's résumé.
Same Language Dub: Ken Watanabe dubs himself twice in the Japanese dubs of the film. (One for the theatrical version and another for the video release)
Shrug of God: Basically Nolan's entire stance on the ending. In his own mind, though, the ending is real, but he feels this shouldn't be everyone's interpretation.
I've been asked the question [about the ending] more times than I've ever been asked any other question about any other film I've made... What's funny to me is that people really do expect me to answer it.
"You mustn't be afraid to dream a little bigger, darling." According to an interview with Tom Hardy, he was supposed to use Arthur's name and "darling" just slipped out. Nolan liked it, so it stayed, even getting trailer time. The slashers rejoiced. One assumes that was unintentional.
Arthur kissing Ariadne apparently wasn't in the original script, so one wonders what inspired Nolan to give it a shot...
Nolan likes toying with shippers, of both the slash and het varieties, apparently.
Viral Marketing: In the Nolan tradition, there were several campaigns, including anti-Mind Crime posters marked with QR reader marks containing links to viral websites.
What Could Have Been: James Franco was originally cast as Arthur, he had to back out because of scheduling conflicts. Nolan's first choices for Ariadne and Browning were Evan Rachel Wood and Don Johnson, respectively, but both declined.
Word of God: Nolan himself claims that, as far as he's concerned, Cobb is in the real world, but that the real meaning of the scene is that Cobb has left his totem behind and that it no longer has any meaning for him. ...clarifies nothing. However, he himself admits he's biased toward the "real world" ending since he's a father himself, and points out that interpretation of the ending may fall along those lines. The fact that the guy who wrote and directed the movie only has an interpretation of the ending serves to underscore that there's no definitive answer to the question of whether the top was going to stop spinning or not.
When interviewed about his role, Michael Caine asserted that any scene that included him was one guaranteed to be in the real world. This either explains the film or makes the Mind Screw even more confusing.
A costume designer stated that the last scene showing Cobb's children in fact used different children from an earlier scene and they were wearing slightly different shirts, which suggests that the final scene isn't a dream.