Here's an excert from the journal entry from one the editors thats in Japan for the PSP launch...
"Sony has rated the battery life to be around the six hour mark when playing games. So far, that seems to be accurate. Games--at least the ones we've seen so far--seem to be pretty good about only spinning the disc when needed. Movies, however, are streaming off the disc at a pretty constant rate, so it seems like you'll get less movie time than game time, but exactly how much less is a little tougher to figure. I would also imagine that the lower power requirements of reading the memory stick means that movies playing off of a stick wouldn't take as much as reading a movie from a UMD. We'll be putting that to the test soon."
http://forums.gamespot.com/gamespot/journal_entry.php?board=909092203&topic=18017788
"Sony has rated the battery life to be around the six hour mark when playing games. So far, that seems to be accurate. Games--at least the ones we've seen so far--seem to be pretty good about only spinning the disc when needed. Movies, however, are streaming off the disc at a pretty constant rate, so it seems like you'll get less movie time than game time, but exactly how much less is a little tougher to figure. I would also imagine that the lower power requirements of reading the memory stick means that movies playing off of a stick wouldn't take as much as reading a movie from a UMD. We'll be putting that to the test soon."
http://forums.gamespot.com/gamespot/journal_entry.php?board=909092203&topic=18017788