Fighting bad design in Apple products one thing at a time.
The blog went down with a Posterous boat and all the recent posts are lost. I am looking into switching to a self-hosted solutions meanwhile. Sorry for the inconvenience.
Actually I don’t think anybody would want to downgrade their 3GS, mine is working fine with iOS4, but 3G suddenly became unpleasant to use. Also 3GS downgrade is a tricky one, so I wouldn’t bother with it.
If you’re lucky, you’ve got a backup saved from before you upgraded to 4.0 (you can select different backups from the drop-down menu). If you don’t have a pre-4.0 backup, you’re unfortunately out of luck—iTunes won’t let you restore data from a 4.0 backup to your downgraded 3.1.3.
Everything went quite smooth for me, here is my iOS3 downgrade success story. The only thing, better do this now, while you still have your iOS3 backup in iTunes.
Read the full guide at lifehacker.com