A company in the US called A123Systems recently developed a very high performance Lithium Iron Phosphate battery which they call the "ANR26650M1". (Click here to view the datasheet.) This battery currently powers Killacycle, the world's quickest electric vehicle. A123Systems are currently moving into the automotive arena with their batteries, including a recent agreement with General Motors to co-develop a battery system for the Chevrolet Volt project.
Unfortunately, as is so often the case with such things, it is not easy for the general public to get their hands on the cells. You can sign up for an evaluation pack which gets you 6 cells for US$120 - not cheap! Fortunately, if you're handy with a screwdriver, there is an alternative..
Power tool manufacturer Dewalt is one of the first commercial users of this battery, having used them in the DC9360 battery for their cordless power tools. Each pack contains 10 cells, and is usually available for around US$100 on eBay (or similar). The first crowd to start taking these packs apart were the radio controlled aircraft enthusiasts, for use in high performance planes. Even at US$10 each, pricing is still a bit high for use in BEVs (its almost three times the price of ThunderSky, for example!), but the technology is perhaps a glimpse at things to come in the battery arena!
Generic disclaimer: This is for information purposes only, perform at your own risk. And it will definitely void your pack's warranty! So don't sue me :)