If you work in D6, I would suggest starting with BrowseCap http://drupal.org/project/browscap , it integrates with several different modules, but is also fairly easy to use.
As for mobile themes, good luck. I did not find one ready-made option, which was even a good starting point. I noticed that the development of the more popular ones was really gone. I think most people do what I do myself. Start by watching a really great WordPress theme for mobile and drop your own.
Aaron source share