Update: Some of our Twitter buddies have noted that you should be able to use the TinyUmbrella tool to kick out of the 'error 1' loop that results after restoring an iOS 5 iPhone back to 4.3.3, and proceed safely from there. Needless to say, your mileage may vary.
We knew the pre-release builds for iOS 5 would be issued to the developer community today, but AppleInsider warns that this comes with a caveat. Apple has apparently told testers of iOS 5 that any devices updated to that beta cannot be downgraded back to iOS 4. They will only be able to further upgrade their device and eventually install the final iOS 5 release.
This note was delivered to developers alongside the latest builds of iOS 5 (build 9A5220p), iTunes 10.5, Apple TV Software beta and the Xcode 4.2 Preview (build 4C104 for Snow Leopard and 4D5031b for Lion). Commenters on the AI forums note that this is not necessarily a new condition for iOS 5; as far back as iOS 3, downgrading was not officially supported.
For larger developers this should not be a major problem, but for smaller/indie devs that use their development devices as their primary or day-to-day devices... well, let's hope that beta 1 is stable enough for regular use.