I wanted to offer a 'lighter' theme for mobile visitors so I tried out the Mobile Tools module http://drupal.org/project/mobile_tools
I got it to work for me after I made some customizations.
It is very good module that has a lot of options...
The method that appealed to me the most was an option to display a block asking the user if they would like to switch. It even detects if its a mobile user browsing the desktop version. That way you don't have to have the block up all the time.