A long-running favorite free theme for Magento called "HelloWired" has been brought into the responsive design ages with a new release of version 3.0 - check it out at http://www.hellothemes.com/themes/hellowired/ and try it today, especially if you are a user of the original 1.4 version from a couple years ago.
Hello Themes has also updated their documentation for this theme, a liong-standing problem for users of prior versions that had a difficult time finding support. Although the 1.4 version seems to be stranded, the 3.0 version now seems to have fairly detailed documentation and a bit of chatter in the community regarding some of the basic installation problems for this theme.
Check it out on a mobile phone, it looks great! Here's a demo store running the HelloWired Magento Responsive Theme: http://VegasHistory.net