I heard some complaints about the latest Facebook iPhone 4 app (2011-10-11) was responsible for deleting user files stored on the phone.
Be warned (and Apple does clearly warn you during the install process), iTunes 10.5 will erase everything on your phone before installing the new iOS 5 Gold Master (9A334).
Check for Updates and download iTunes 10.5, then install it.
Disable 3G and wi-fi, so no additional data is captured to the phone between your backup’s execution and the actual device reformat.
Do a wired sync of your iPhone or iPad to iTunes 10.5.
Do a Transfer Purchases to be safe, too. The idea with iOS5 is that you get on the iCloud, so theoretically it won’t matter, but as I said.. play safe.
Follow the install path.
You will need to re-enter all stored passwords (reasonable security measure). You have all of this easily accessible, right?
Finally, the one key configuration to check out is what email address is associated with your iMessages account. You can find it in the settings for the app, in the settings app. (get it?)