No, Jim Carrey is always the lead role, or at least the joint lead role. (Huge fan of his work, by the way!) This is his first film since Batman Forever in 1995 in which he hasn't played at least a version of the main character. He's Colonel Stripes in the upcoming Kick-Ass sequel, so maybe he's finally taking his foot off the gas after all his $20 million-a-movie roles over the years. Saw this film today-pretty good, I thought!
Kick Ass 2 comic (and presumably movie) spoiler relating to Jim Carrey's character:
As for The Incredible Burt Wonderstone: I saw it on Sunday and really enjoyed it. My expecatations were fairly low after all of the mediocre reviews, but it was actually very enjoyable, Carrey was back to his manic best.