For the upcoming 3DS, battery life when running a game in 3D, is only 3-5 hours, with some warning that the actual time may be closer to the lower end of that range unless Wi-Fi is turned off.
Battery life is a measly 5-8 hours when running regular DS games.
President Satoru Iwata said back in October:
As for the battery, it is inevitable that Nintendo 3DS will be a device which requires more frequent recharging than Nintendo DS. This is why we are going to include the cradle, which is a dedicated battery charger. Perhaps we may need to dispatch to our consumers a message, something like, 'Please place your Nintendo 3DS on the cradle as soon as you return home with it.'