How to revert to iOS 4.2 from iOS 4.3 beta?
It seems you missed the "Read Me Before Downloading" section at the top of the 4.3 beta page:


  Devices updated to iOS 4.3 beta can not be restored to earlier versions of iOS. Devices will be able to upgrade to future beta releases and the final iOS 4.3 software.


According to Apple, anyway, no, you can't revert to 4.2.