I just got back from Cali (LAX), flying out of Dulles (D.C.) - no problems and still had over 20% battery left, though I didn't use my MBP every single minute of air time either (I'd say I used it 80-90% of the time). I got tired mid-flight and took a 20min snooze, but I watched a full length movie, listened to music and went online (my flight had internet service available for a few bucks).
Tips:
Before you leave for airport - fully charge your MBP and turn it off (not sleep). Turn it back on once up in the air - avoid using it in the airport while you wait to board, to maximize your battery once flying.
You don't need 50% screen brightness in an airplane - go lower (get a window seat if you can so you can shut the window during daytime flights).
Turn wireless off.
Turn keyboard backlighting off (if you're not typing).
Quit any programs you're not using and single task instead of multitasking.
You might be able to squeeze in 2 movies, depending on the length of the movies, condition/life of your battery and screen/keyboard brightness settings.
Of course, my flight also had power outlets...the girl next to us simply plugged her 17" MBP in the whole flight lol