Now that iOS 5 Beta 4 is out, Apple has released the iTunes 10,5 Beta 4 along with it.
The beta works fine on the iPhone 4, but many people have had problems with syncing the beta to their iPads. The message the appears like this:
The problem should be fixed if you download iOS 5Beta 4 on your iPad.
If the problem persists, you may have to back up your files and then do a master reset on your iPad: Settings > General > Reset > Erase all Content and Settings.