Gosh, I'd intended to start a thread but somehow hadn't got round to it..
We've just got back from 2 x 13 hour flights with a 3 year old. Worked far better than I had dreamt, partly because we had discussed it LOTS before we left. I know this won't help those of you with 'proper' toddlers who can't be reasoned with but we explained we would get on the plane, play for a while, eat a meal then she would have to put her pyjamas on, have a wash, clean her teeth, have story, then go to sleep. She was so familiar with this by the time we left, she could bore for England but it worked ! She slept for 10 hours on the way out and 7 on the way back.
For awake times I would recommend:
personal stereo & story tapes
wipes to clean the windows & tables with (don't laugh, passed a lot of time)
pencil sharpener which collects bits (or use the sick bag)
A pencil case with just a few pencils or crayons, a ruler and a shape stencil
Crayola activity pad
Sticker books
PVC type stickers which can be reused (good for sticking to those very clean windows..)
Mothercare playdough, doesn't dry out and comes in small tubs
Aircraft toilets - top banana, just make sure you know how to cancel the steward call button...
Also take advantage of kid's hand luggage allowance and take clean tops & knickers for grownups as well as total change for small people so at least you don't look as bad as you feel at customs.