I've read some of the information available on the Kindle Fire, and I've come to the conclusion that the Kindle will live or die by its battery life. This is because the Fire is placed directly at the high end of the Kindle line. Since the Kindles are e-ink screens, with little in the way of interaction, they have a battery life of days and weeks. With an LCD screen and a touch sensor, as well as a browser and video playback, the Kindle Fire will not be able to compete with them. However, if it does not have at least the battery life for a full day of reading, then it will fail to successfully live up to its market niche.
From what I'm hearing right now, the Kindle will have enough juice for 8 hours of continuous reading (slightly less for watching video). That is what I consider the absolute minimum. If it fails to live up to that duration, then it will soon find itself going the way of other fancy, but less than useful, gadgets.
This has not stopped me from adding the Kindle Fire to my wish list, of course. If anyone's interested in giving me one for Christmas, I'd be happy to test out the battery life for you and report it here.