When you send your iPhone off to the wonks at Apple for repair, it will come back wiped. Do a full backup of the device in iTunes. Messages, apps, data, everything, before you send the device away. Then restore it from that backup after the repair, and you should be set. Instructions are likely buried in the Tunes help files somewhere. If you can't find 'em, post back and I'll help you with it.

I want to back up my SMS messages from my iPhone onto my computer (mac)
Does anyone know how you can do this? I recently just dropped my iPhone and the screen smashed, but it still works absolutely fine. Im worried that when i send it off to be fixed that messages will be cleared. I've got lots of messages i would really like to keep. If anyone could help me