I'm not sure this will ever get off the ground, but you've got to admit, a battery that has the ability to free-load off another battery within bluetooth range is pretty cool.
Here's what Chen at Gizmodo had to say about it:
Motorola Corporation has come up with a clever way to keep friends and family members in cellphone contact even when their batteries have gone flat. The new system takes advantage of the fact the most modern phones can use Bluetooth for short range wireless links, which requires far less power than a normal cellphone call.
When a phone senses that its battery is running low it disables all non-vital functionality and blocks in-coming calls. At the same time it switches its Bluetooth transceiver to discovery mode and searches for compatible "proxy" phones nearby.
The failing device then uses a nearby phone to send a warning text message, via Bluetooth, to everyone on a pre-arranged emergency list. This lets them know that they can still get in touch, but only by texting. With the permission of its owner, the proxy phone relays any incoming text messages via the normal cellphone network and passes them on via Bluetooth.