WARNING to biteSMS users who upgrade to ios4!!
If you are currently jailbroken and using biteSMS as your default app, and you upgrade to iOS 4 GM, do not restore your backup, as the stock SMS app will be broken.
I think it is because biteSMS disables the stock SMS client and iTunes backs up this information so you won't have any SMS app. I'm restoring as a new right now.
It may be possible to disable biteSMS before you upgrade and then restore the default SMS app as the default, but I have no way of trying this now...