$10 shipped gets you one of these >>
http://cgi.ebay.com/Replacement-for-...ayphotohosting
I currently have a 2600 li-poly and it works well. The current setup of the 6700 does not allow you to maximize battery charge with these polymers so you'll be getting maybe 85-90% max charge efficiency, but the the low prices make a worthy trade off.
I've also noticed with Li-Poly batteries as opposed to the normal ones are lighter and don't heat up.
The problem is that people have reported the 1500's just barely fit with the stock battery door. If anyone knows a solution, that would be great.