A recent MIT Technology Review article points to an emerging new battery technology. This looks interesting. Making Li-ion batteries safer and more compact has a variety of interesting potentials. The growing use of them in portable tools is just the first one. It's too bad A123 does not provide more detail on what the battery characteristics really are. As best I could guess, the energy per kilogram is not significantly different from other Li-ion (about 200Wh/kg) but the volume is much smaller. For applications like automobiles volume matter much more than weight. Tools need the high peak current capacity that they also brag about. If you have a dependable peak current source available there are various efficiency optimizations available for motor design, hence the interest by tool makers.
This one is worth watching.