I have a special kit for flights. I usually take an aisle seat so I can have enough room, and buy my neighbor a beer just to keep them happy when I bump them! Keep your neighbors boozed up and they won't bother you.
My main performance controller for flights is an Alesis Vortex. The Vortex tends to take up a lot of the aisle (I play with my elbow out a little more), but it's easy enough to lift it up when the flight attendants walk by. Plus, the Vortex has an accelerometer, so when I have to lift it it encourages a bit of spontaneity during songwriting.
Since I have a 20" Sony Vaio, I also bring out one of those Odyssey laptop stands. That allows the APC 40 to fit underneath, and lifts the computer up enough that it sits above whatever my neighbor has on their tray table.
I also have some battery-powered speakers I bring out if I'm seated in business class: http://www.tivoliaudio.com/products/por ... black.html
I engineered some handy clips out of old over-the-door clothes racks. They hang perfectly on the seat in front of me. Usually the person in front doesn't mind, since I also duct tape some foam to them like a little pillow. The speakers sound OK, but airplanes are noisy. On certain kinds of planes I have to turn them up pretty loud.
For an audio interface, I just bring a MOTU Traveler. Pretty portable little unit, and fits right in my lap. If I plan to record vocals, I also have a cheap AKG Perception 420. I use one of those round pop filter things. The mic stand base goes on the floor, and the stand pole part fits perfectly between the tray table and the seat in front of me.
So yeah, working on music during flights can be VERY productive. I'd just encourage you to keep it to flights over 90 minutes since sometimes it takes a few minutes to get all the gear set up! I'd also recommend a rolling case for everything. Make sure it's wide enough to fit the Keytar, and make sure to be the first in line to board since you will need the room in the overhead bin for the case after you unpack everything.