Originally Warren was known as Angel on the original Xmen team, later on he became Archangel (I think once he joined the Four Horsemen, but there's a good chance I'm wrong). I've been told that the chick named Angel in the movie is some mutant that was much more recently introduced. I guess since the only original member they got right was Beast that they'd at least have another female mutant on the team who has the correct name but wrong everything else.
Also, being that Russia was involved this was the only chance to get an Omega Red appearance. I mean, the storyline was already fucked to hell so why not at least a cameo?
@ Liverslapper: I wasn't even aware that there was a comic called First Class, I just assumed the movie was supposed to be based on the original Xmen team (Cyclops, Beast, Angel, Ice Man, and Marvel Girl). Are the First Class comics much different?