Henry Cavill - though he is getting a little old to play a 17 year old. Some other thoughts could be Hayden Christiansen, or even some unknown!
Rachel Leigh Cook. This is also who I picture when I read the books. She is so petite, and beautiful - and she looks like she has a mischevious side :)
The rest I really haven't given much thought. You know that guy from X-men who played that giant, muscular kid? I think he'd be fantastic playing Emmet. And Rosalie just has to be a knock out blonde!
If you haven't picked up these books - do it now! So much fun to read, not too much thinking involved, and they will draw you in!