According to the BBC film news Beast has been cast in X3:
"Kelsey Grammer - better known to TV fans as Dr Frasier Crane - is joining the cast of X-Men 3 as Beast, a giant, blue-furred mutant monster. "Kelsey was born to play the role," says Marvel Comics producer Avi Arad, explaining that Beast is a pacifist PhD genetics researcher who turns into a big blue monster. "This is a big role in the new movie, and when we met with him, you can see that Kelsey has that you'd-better-have-a-dictionary-handy intelligence."
Grammer's not the only recent addition to the franchise's mutant roster as new director Matthew Vaughn (Layer Cake) has been fleshing out the cast list. Ex-footballer Vinnie Jones is playing Juggernaut, a 6 foot 10, 900-pound human battering ram, and Maggie Grace (from US TV's Lost) is preparing to play Kitty Pryde, a woman who can walk through walls. The role of winged mutant Angel has yet to be filled!"