iOS 5 beta 2 was released yesterday. For those developers who have already installed beta 1, you may see this message after installing iTunes 10.5 beta 2: "The iPhone XXXXX could not be synced because the sync session failed to start".
The reason is that iTunes 10.5 beta 2 can't sync/back up iOS devices that have 10.5 beta 1 installed. That is, the iTunes version number and your device's iOS version number must match. So before you install iTunes 10.5 beta 2, make sure that you back up your iPhone/iPad/iPod Touch first.
How to back up your iPhone/iPad/iPod Touch
Launch iTunes 10.5 beta 1 and connect your iPhone/iPad/iPod Touch.
In the left pane, under Devices right-click on your iPhone/iPad/iPod Touch and select Back up.
What to do if you've already installed iTunes 10.5 beta 2 and haven't done a full backup of your iPhone/iPad/iPod Touch yet.
The most important thing is to downgrade/revert back to iTunes 10.5 beta 1. The good news is that it is easy to revert back to iTunes 10.5 beta 1. First try re-installing iTunes 10.5 beta 1. If this doesn't work (for example iTunes keeps crashing), un-install iTunes properly by following Apple's Support article on "Removing iTunes for Mac OS X". Note that removing iTunes won't affect your music, videos etc in your iTunes library:
iTunes library content such as music, videos, and iPod games are saved in your Music folder by default and are not deleted by removing iTunes or it's associated files and folders
Once you have removed iTunes, you can re-install iTunes 10.5 beta 1 and sync/back up your iPhone/iPad/iPod Touch.
Next steps: install iOS 5 beta 2
Download iTunes 10.5 beta 2 and the appropriate iOS 5 beta 2 for your iOS device from the Member Centre and then follow the instructions detailed in the Guide to installing iOS 5 beta, step 3 onwards.