Man, how did I miss this?! Worx has an autonomous, battery-powered lawnmower, and initial reviews are pretty positive. The price has really come down, too: list price is only $1000.
Sigh. On the other hand, I don't have a $1000 lying around for something like this. Also, previous experience with robot vacuums was very disappointing after the first year.
Guess I'll just keep my eye on it.