Devices updated to iOS 5 beta can not be restored to earlier versions of iOS. Devices will be able to upgrade to future beta releases and the final iOS 5 software.
Yes you can, I just did it a couple of days ago. I already had iOS 4.3.3 downloaded. With the latest beta version of iTunes on windows I held down shift and hit restore, pointed to my download of 4.3.3 and the rest was history. No dfu mode or anything.