I\'m using an 3rd party application for device detection called FiftyOne Degrees (http://51degrees.mobi).
That application uses WURFL (http://wurfl.sourceforge.net/)
You may also need to rebuild FiftyOne.Foundation.dll, source can be found here:
http://51degrees.codeplex.com/
With the new dll in place and the updated web_browsers_patch.xml in the App_Data folder I was able to resolve my FF4 mobile redirection issues.
This is the XML I used:
I was using an old version of the DLL and the XML alone was not enough to get FF4 recognized.