The BP-5L battery in the N800 is 1500mAh. The battery in the N810 uses BP-4L which is supposed to have the same capacity. So both of the N8x0 models have a higher battery capacity than the N900. Due to the, I believe, improved battery saving capability of the OMAP3 chipset this shouldn't matter too much, except that the GSM/HSPA part of the N900 will suck some juice no matter what.

EDIT: I've found some sites selling genuine Nokia BP-5L batteries and claiming they are 1300mAh. However, Nokia's web pages say 1500mAh, and that fits with what I've heard before and I'm inclined to trust the Nokia site more.

770/N800 battery:
http://europe.nokia.com/find-product...-battery-bp-5l

N810 battery:
http://europe.nokia.com/find-product...-battery-bp-4l