Battery Crisis

Li-Po's cost too much for hobbyist robotic enthusiasts (mostly). Also they need constant monitoring of the cells so that it wont blow up destroying the whole robot. Robots tend to be a lot more expensive than RC equipment (depending on the robot of course). And the miniature weight of the cells is normally not needed. If they are, then the robot is poorly designed and the mechanics should be redesigned.

Check out FMA Direct. They are a bit expensive, compared to regular NiMh's or NiCads, but weight aside the physical size is an advantage. They also sell discharge controllers which sense cell voltage and activate cut off circuitry before the pack can be damaged. The packs and the chargers are designed to work together and the seperate charge cable from the discharge cable feeds into temperature/voltage sensative voltage elements which automatically ballance the packs.

www.fmadirect.com

If you need, or want the highest possible amount of energy storage in the smallest amount of space and weight lipoly's are the way to go. As bloody-orc stated though they're pretty pointless for your run of the mill line follower or bounce off the walls and wander around robot.