Personally, I can't concentrate hard enough on a plane to do any real development work (and besides, I need at least 2 monitors to do any real work these days!).
However - time on the plane / train can be great for doing all of the things you have to do that aren't programming! Organising task lists, updating bug lists, drafting proposals for clients, etc.
I also find it a great opportunity to catch up with some of the less important e-mails that I have saved up, along with all those interesting RSS feed articles I've been saving for a 'spare' moment.
In fact - it ties in with a great trick I was taught for helping to not get distracted at work. When you get e-mails or RSS feed updates - check to see if they need to be read now - and if they don't - just file them away somewhere safe. Then - next time you have a few minutes and you can't think what to do (or you're on a plane!) - you'll have a stock of interesting things to catch up on!
Just a thought! ;o)