I was instantly unsure about Anne Hathaway but I'm starting to warm to the idea after seeing her in The Devil Wears Prada; she looks dowdy but still pretty. Also, as others have said, they can't have someone too 'plain' or it wouldn't be Hollywood! And I got the impression that Emma would be quite attractive, just an ordinary girl.
As for the novel, I really enjoyed One Day, and - again, like many people - found Dexter horrendous sometimes! The leeway he gets from Emma was just infuriating enough to dent a 'perfect' reading of Emma, which I think is something lazy that is far too common in good books (ie the do-no-wrong-protagonist).
One thing I did notice, and I'm going to try hard not to spoil anything, but I didn't think the start of the book really set up what unfolded through the rest? I know it was explained more at the end, but I just didn't feel it until I read on.