I ran mine out of battery for the first time today! That was with a full charge over night - but I'd loaded on a bunch of new books (20ish) via computer transfer last night, then downloaded a fair amount of samples in the morning. I read about half a book with the wireless off but it locked itself down a few minutes ago.
I don't have an SD card installed, but I'm pretty sure it's simply that indexing is hugely battery intensive - and a new note to myself to not do any bulk downloads!
I blame fictionwise's great sales, which led me to stock up on a ton of books over the last few weeks.