Brotherhood is not the only instance of Gundam and Fullmetal Alchemist sharing several voice actors, the original anime also featured several seiyuu from Gundam SEED. So it's probably a safe bet that if there is a FMA anime, then they will most likely follow after a Gundam series, and have several voice actors in both.
The Cameo: Aaron Dismuke, English voice of young/teen Al in the 2003 series and The Conqueror of Shamballa, appears in a brief role as Young Hohenheim as he couldn't reprise the voice of Al (due to his voice obviously breaking).
Ling Yao is the son of the Grim Reaper, while Shou Tucker spends his spare time dissecting living things and swinging around a Death Scythe who happens to be Ed. Along with this, Hawkeye wears an eyepatch and is in love with Tucker. In the meantime, Roy Mustang doubles as an ineffectual magical werewolf and Selim Bradley is a flamboyant blue-haired ninja. Major General Armstrong is a spider, and Winry has a flame-throwing cheese grater for a weapon. Kimblee enjoys listening to loud music while Scar has gears on his legs for weapons. Guess what? They're still enemies. Meanwhile, Father is also an undead teacher in his spare time, Greed is a transforming legendary sword that's annoying as hell, and Lust is suddenly... flat-chested. May Chang grows up to be a chain scythe than can transform into numerous ninja weapons, and Alphonse Elric doesn't knowhow to deal withthis. He also spends a lot of time getting beaten up by Lieutenant Colonel Hughes.
Don't forget that Death is also Hohenheim as well as Tamaki's dad (Ouran's Chairman).
Trisha Elric's Japanese voice actress has now moved on to voice Carla Jeager. Funny how that does not bode well for her.
A lot of the dub cast (especially in Texas) got their start from Dragon Ball Z.
Father used to be more interested in creating the perfect being via robotics than becoming it through hermetic magic and still had at least one rebellious creation. Tucker, Ross, and Breda were among those rebellious creations; in fact, Tucker killed Father in cold blood not too long after we first saw him. Ross eventually got together with Hughes, ironically. Major Armstrong used to be…okay, everywhere. Dolcetto once had a third eye.
For therecut, obviously some places changed. Alex was…a few less people, Hawkeye took Ross's place, and Edward and Knox were members of the Ginyu Force. Alphonse became a Namekian, while May got blue hair in her thirties while becoming far less badass and even brattier (she did invent some good stuff, tho).
A lot of the VA's for this anime are in Fairy Tail now, too. Ling is Natsu, Elicia Hughes is Lucy, Barry the Chopper is Lyon, Lan Fan is Sherry, Hawkeye is Erza, Major Armstrong is Elfman, May is Mirajane, Scar is Simon, Justin is Loke, Sloth is Laxus, and Fuhrer King Bradley is Jose. To name but a few.
What Could Have Been: Lan Fan's actress wasn't available for the final episode of Brotherhood, meaning we miss the end of her character development from the manga.
The dub had a surprise cross-region one late in the story: Scott McNeil as Hohenheim!
Earlier than that, Johnny Yong Bosch, typically a Los Angeles-based actor, makes a surprise cameo as a supporting character for one episode.
The series is purported to be one of, if not the only anime series to be dubbed by actors hailing for all four major dubbing cities: Dallas, Los Angeles, New York, and Vancouver.
It's easier to list Texas-based voice actors who haven't been in Fullmetal Alchemist at some point than those who have.
In the Japanese version, an amazing amount of the cast had been involved in Mobile Suit Gundam SEED, which Fullmetal Alchemist had immediately followed in the popular (at the time) Saturday 6 p.m. timeslot after SEED had finished airing. The tradition would be continued with Mobile Suit Gundam 00 and Fullmetal Alchemist Brotherhood.
In Germany Roy is Johnny Depp (voiced by David Nathan who is the German voice actor for most of Depp's stuff.)
Hungary: There are quite a few veterans from Total Drama: Courtney is Lust, Brick is Edward Elric, Noah is Denny Bloch, Trent is Vato Falman, Tyler is now Solf J. Kimble, Scott is Kain Fuery, Mike is Brotherhood!Envy, Chef is Brotherhood!Sloth, Anne Maria is Paninya, Blaineley is Rebecca Catalina, Bridgette is Izumi Curtis, Jo/Eva is Gracia Hughes,
Hoho and Hohopapa for Hohenheim, Scarbro for Scar's brother, Sexyhobo/Gar for Scar (no, really!), Shinri-kun or Truth-kun for the guardian of the Doors of Truth, Greedling or GreedLin for Greed once he takes over Lin/Ling's body (which is used by Ed as a Shout-Out to the fanbase at one point), etc.
Also Bob the Blob/Proto-Father for Father's original form, Mr. Monopoly, Colonel Clean (a.k.a. Sparkleman), Palm Tree, Shota McShadowrape, Ninaxander, the Pimp Suit, Fetus!Envy, Godfather, Scarbro..
What Could Have Been: The original, three-hour-long screenplay for Conqueror of Shamballa featured (among many other things) a scene at the end involving Mustang and Winry meeting in front of Hughes's grave and reconciling their differences. This was cut from the final draft and the equivalent scene moved up to the first half of the film, meaning that the last time the audience ever sees Mustang is during the climax, leaving his character arc feeling jarringly unresolved.
Many other elements were cut too. There was supposed to be a love triangle involving Ed, Alfons, and Noah but it was scrapped because they felt it'd make Al's story feel unnecessary. Izumi's death scene was cut, Noah had more (or older?) children, Gracia had a slightly larger role (with a far different personality from her Amestris counterpart too), and Winry had more lines.