Hehe, very nice - I'm a big fan of Wallace and Gromit, the Wallace you have here looks great!
I think the face shape on the first vowel sound (as he says 'Off') doesn't look quite right, though I'm not too sure why. It's either because you can see both his upper and lower teeth, or just that it's not quite the shape that the guys at Aardman animation use for Wallace. (I think thay have the cheeks dropping right down, as you have here - but a much wider O shaped mouth)
As you say, it's for a lip syncing exercise, but for fun it'd be nice to see this rendered on twos (take out every other frame) to give more of a stop frame motion feel to it.
I'm guessing you checked out the Aardman site, but for those that haven't seen it One of the best sites I've visited for a while, (the flash site that is) Click on the little characters at the bottom as they pass by - lots of fun