Sony's from as far back as cassette walkmans barely bigger than a cassette case (which I had) upto todays mp3 players always seem to be offer great battery life.
I have a 2gb Sony NW-805A where the battery always seems to last 14hrs+ on a single charge & also recharges (via usb) fairly quickly too.
So definatly worth checking out the Sony range if battery life is your main priority although it has to be said I do get annoyed that iPOD sheep do have a vast array of docking stations and other addons for their mp3's compared to mine but then again I've not heard of any Sony batterys giving up the ghost shortly after the warrenty runs out