Well, to have something to compare to, has anyone's DS Phat or DS Lite batteries died out? My Lite is still going good, REALLY GOOD, and it's probably gone through at least a thousand charges.
On the other hand, my old Ipod Touch, which has had FAR LESS charges, is on its death knell. When full charged, it lasts for about 2 hours.
FYI, you can buy replacement batteries from Nintendo directly. They cost $15 each. Their online store is stupidly difficult to browse, though. I know. Surprise.