I started to use my Kindle 3 on the 12th Oct, and so far charged it *once* (on the 6th Nov.)
It's been on, including Wi-Fi all the time except for 7 hours without Wi-Fi (in an aeroplane) and always left without actively "switching off".
I've downloaded about 100 books solely on Wi-Fi and now it shows (well, I don't know how many "bars", but) about 80% charge.
I've read 4 books and demonstrated it for 10-15 people, with a lot of "show-off" features.
I've bought 15-20 books on the Kindle directly and the rest via PC and/or mailed to it.
Only 3-4 books have been sideloaded.
So, for my part - the battery life is non-complainable