I can't comment on how Lion's FileVault 2 affects battery life, but I've had it running (Full Disk Encryption) on an older (2007) MacMini with 3GB of RAM for a few months, and it is a great solution for keeping a computer that might get stolen secure.
Beyond the initial encryption, which took a few hours, I really haven't noticed any performance difference. Given that this is an older machine, any negligible difference I might have missed would be even smaller on a newer machine like yours.
I was curious about this myself at the time and found this AnandTech article, which boils it down to a negligible CPU load, and a slight hit to your hard drive's throughput. They were using an SDD though, so the results may not exactly correspond with a spinning disk.
So if you're reading and writing to the disk a lot, you might notice a slight decrease in battery life as reads and writes will take a bit longer, but I doubt it'd be anything too drastic.