I know what you mean about Michael Cera, since he ends up being typecasted in every movie he has ever been in. Just look at SuperBad, Juno, and Year One for example. He was a socially awkward teen in all of those movies. Though I heard Youth in Revolt was a different take considering he had a bad ass counterpart to counterbalance the socially awkward kid he was playing.
While Scott Pilgrim isn't an exception, since Scott is basically a socially awkward teen archtype, except he's in his early 20's, I have to admit he is perfect and the trailer also convinced me that Cera is perfect for the role, to the point that I didn't mind it was Cera playing Scott.
Good to see another Scott Pilgrim fan around these parts. As you can tell by my avatar, I am looking forward to Volume 6, along with the Movie and video game as well. Heard good things about the movie from people that saw the test screening.