Actually 7805 can supply 1A (with proper heatsink) which should be enough, as the charger that I use for my N810 (not 100% sure it's the one it came with, just some Nokia charger I had laying around) supplies only 890mA.

I'm also going on two long flight next month (there and back again and pondering how to best handle this, my first thought was to hook up a step-down transformer (aka buck-regulator) to my RC helis battery (11.4V 2100mAh) which should provide plenty of juice, OTOH charging that battery on the other would mean taking the special balancing LiPo charger and a 12V power supply with me so maybe it's not such a great idea afterall... (unless I take two battery packs, in which case there is no need to recharge them). There is also what security thinks of the battery packs as to keep weight down they're not in metal casings, just shrink sleeve. They do look a bit suspicious...

Then a quick calculation about using simple AA batteries; rechargeable ones are typically 1.2V and 2100mAh, which means 4 is either not enough or just barely enough voltage when they're full to the brim; a boost-regulator (step-up transformer) would be needed. Alkalines are 1.5V and thus can provide enough voltage to charge for a while even without the booster.

Maybe I need to go and check out what the shops have to offer before starting to build some contraption that will get me "special treatment" at every security check along the way...

ps. LiPo is just as dangerous as Li-Ion when it gets damaged. In fact anything with similar energy density is inherently dangerous.