Ascended Meme - Fans noticed that Ritsuko's outfit resembled the uniform of Lawson's convenience stores, and it became a meme. Bamco and Lawson's noticed the meme, and started a cross-promotion campaign that is still ongoing; you can apparently buy raffle tickets for Idolm@ster goods at Lawson's.
The Faceless - The Player Character. Also, Mr. Takagi. Actually, aside from Kotori and the Jupiter trio, every other character you come across, including the satellite characters on the girls' routes, are all faceless.
Fun with Acronyms - The MASTER ARTIST CD series has one hidden. If you look at the covers of the CD jackets of each album, every girl is saying something, most of them in English. If you put them in the correct order, they spell...
No Export for You - While the console games at least require the Japan region consoles, THE iDOLM@STER Mobile i takes it to the next level. Unless you have your iPhone "modified", you have to live in Japan in order to play it.
Finally averted with the Puchim@s anime and the Shiny Festa games. The former was simulcast by Funimation and the latter received translated ports on the iOS App Store.
The 765 in 765 Studios is pronounced "na-mu-ko", from "nana" (7), "mu" (6, for counters), and "go" (5, where "go" and "ko" are the same character, mostly). If you still don't get it, this is how Namco is pronounced in Japan. This reference can also be found in other works by Namco, the Ridge Racer and Ace Combat series being good examples.
Also, the DS game's 876 Production is read as "ban-na-mu", with "ban" (8) and then the "na-mu" part as above. "Bannamu" is a portmanteau of "Bandai Namco", and is indeed a common short form for it.
Limited Wardrobe - A rare case where both apply. With the amount of DLC out there's almost no end to the clothes the girls can wear. But in the in-game communications they are always using the same set, the only diference being that it changes between Summer and Winter.
Trauma-Induced Amnesia - In the alternate Miki route, after she knows that the unit will be disbanded, she is surprised. Days later, she shows up having forgotten the memories between her and the Producer.
The i DOLM@STER DS
Beach Episode: Ryo's story features a chapter at the beach, where the 876 idols have a photoshoot.
It does lead to a Hot Springs Episode when Ryo needs a break from posing in the feminine swimsuit.
Stacy's Mom - Ai becomes a victim of this. Her mother, an ex-idol herself, becomes nostalgic for the glory days when Ai makes it big and decides to make a comeback. Ai is shortly overshadowed in the public eye by her still incredibly hot mom.
Blonde, Brunette, Redhead - While very possible to do by yourself, it's also probably one of the reasons why 961 has the three idols it has - in fact, it's an odd case where the "redhead" of the trio is actually blonde, considering Takane's silver locks. Unless, of course, you count Miki's hairstyle from after she stops dying it, which is probably closer to being red....
Color-Coded for Your Convenience - The Perfect Sun girls (Makoto, Haruka, Yayoi and Hibiki) are red, the Wandering Star girls (Takane, Ami, Mami, Iori and Yukiho) are yellow, and the Missing Moon girls (Azusa, Chihaya, Miki and Ritsuko) are blue. This color code was also used in Cinderella Girls.
Fan Translation - As of November 2012, The Idolm@ster SP Perfect Sun is fully playable in English. Chihaya's route from Missing Moon is also released with Iori and Yukiho from Wandering Star. The people at TLWiki are continuing to work on the other two PSP game routes.
Lover Tug of War - The Producer is the victim of this in Azusa's ending, with her and Miki on each side.