Since owning my iPhone I’ve had this happen to me a couple of times now, I plug in my iPhone to do a sync and recharge and get a message telling me “An iPhone has been previously synced with this computer” and gives me the options of setting it up as a new phone, or restoring from a previous back up. Neither of these options describe what I want, as I don’t have it back up on every sync I would lose too much data, and I certainly don’t want to start fresh.
As it turns out, this screen is very misleading and not as threatening as it may appear. Selecting the option to set it up as a new phone does NOT revert your phone back to its starting point as the message may imply. The only change that is made is you will be prompted to rename the phone, after that your phone will sync and all of your settings and data will be preserved.