Not sure about streaming but I know you can get a video converter to convert some movie files into the right format and put them on an iPad. If you want some suggestions look up AVS video converter which is simple and easy to use since you just open it up and click browse to find and select the file you want to convert and then select the output format.
I've done it before with my iPod and generally they use mp4 format although I'm not totally sure since its been a while.
some airlines have apps that connect to content servers from which you can pick what you want to watch ... check your airline
I'd never know about this since ive never flown but knowing how cheap airlines are their selection of movies if they had some would likely be limited to some really bad choices xD.
true ... however, for example, AA says it has up to 250 movies and 180 tv shows available on international flights so, no matter how bad some options may be, i'd guess that you'd be able to find something worth watching ... you'd have to be incredibly picky not to xD