[gim] gutted some li-ion laptop batteries to replace his roomba's battery pack. He had to pick up a li-ion charger and add a protection circuit to deal with the li-ion cells, but ended up with a new lighter pack for his roomba. If you head this way, the protection circuit is a vital component to prevent fires/explisions/etc. Looks like a great resource for robot power or even R/C projects.
http://www.hackaday.com/2008/03/15/li-ion-for-your-roomba/