Backup your iPhone 3GS in iTunes, then restore that backup to your 4S:
1. Disable automatic syncing on connection in iTunes Prefs > Devices > Disable....
2. Connect the 4S and right-click the divice tab (left border of iTunes window) and select "Restore from backup." You should see a list with a choice of the 3GS backup. You may not, then have to do a full restore by selecting "Restore" on the 1st page, then chosing to use the backup in the last stage. This will also update the phone to iOS 5.1.
iOS 5.0.1 - Jailbroken? If so, a full restore will partially remove the jb, and possibly fail with error 1015 or 3194.
Message was edited by: modular747
Thanks but iPhone3GS has only a few weeks info that I want to keep.
Are you certain that iTunes will not erase the existing 4S back up?
If I back that up and restore it on to the 4S it will wipe out all the months of stuff on the phone that I need to keep.
Question: is it possible to have two entirely different back ups on 1 iTunes account?