Some kind fella went to the trouble of scoping out a bunch of free and premium responsive themes, so I figured I'd post a heads-up to his post:
10 are free, but most of them are rather skeletal, except iTheme2 by Themify (nice!) and a Responsive TwentyTen theme. The rest are premium themes from Themeforest and Themefortress.
Disclaimer - The fella noted above posted a disclaimer to the effect that he gets a commission from some of the affiliate links on his page.
Um... I don't :o)