Stop my old iPhone from backing up tl;dr How can I tell iTunes to not back up my old iPhone 3GS when I plug it into iTunes

I have an iPhone 4S, and an old 3GS I'm using as an iPod touch.  I asked the salesperson at the apple store whether having my old iPhone backing up in iTunes would cause problems, since iTunes only stores one backup.  She said probably, and that I should set iTunes to stop backing up my old phone when I plug it in to sync.

a) Is this correct, that a backup of my old 3GS will overwrite my last backup of my new 4S?

b) Is there a way to turn backups off for a specific device (short of just remembering to click the X when the backup starts)?
(a) iTunes named backups (on your hard drive) using the UDID (Unique Device Identifier) of successfully connected iOS devices. So it's erroneous to state that a backup of a 3 GS will overwrite a backup of another device since each device has a unique UDID.

(b) iTunes 10.5.1 doesn't provide option to not back up. However, you could uncheck the "Open iTunes when this iPhone is connected" which then allows you to connect your iPhone to iTunes without triggering a backup.