In reply to Sharp: another +1 for Sanyo Eneloops, they are THE SHIT. I used several sets in the Himalayas for my headtorch and digital camera on a 5 week trip (with a homemade solar charger), and had no problems at all with them dying on me. Always had a spare, fuly charged set on me just in case, but only seemed to need to change them once or twice over the entire trip. They stay fuly charged for a long time, as already mentioned, but their other main advantage is that they work well in the cold as well. My digital camera was totally fine up to 7000m+.
Don't bother with crap batteries off ebay, it's full of cheap rubbish from China nowadays. Get some proper Eneloops from a decent supplier - there's a few online UK companies, I used 7dayshop.com
As for alternatives, NiMh is the only really practical option at the minute. I did look into lithium technology - non-rechargeable lithium cells are a good backup solution (lightweight too and good in the cold), but there's no AA-sized rechargeable lithium cells around yet.