One problem with laptops is that if you sit in back and don't have an exit row, they're often pretty cumbersome - so if you do get one primarily for movies, etc, while flying, you might want to avoid the temptation for a bigger screen. A 14.1" nonwide is already pushing the limits of useability in coach: on AA, with MRTC, even a little bit of a recline by the person in front of me keeps it from being useable for typing or games (I can usually get an angle where the screen is viewable for movies at least.)

On some of the shorter seats (non-MRTC, JAL) it's pretty difficult to get even an angle just to watch the movie.

I'd imagine, based on the dimensions that a 15.4" widescreen would probably be manageable (it's height is about the same as the 14.1") but a non-wide 15" or a 17" wide is probably right out.

If you can stand it, a 12.1/13.3 widescreen is likely a lot more useable in cramped conditions. I'd also be curious how a tablet-PC form factor machine would do under those conditions.