As of iTunes 8 (only one I've used, could be earlier), iTunes no longer provides multiple backups of the iPhone. It will only keep the last one. This really is a problem if you need to go back in time to find a non-corrupted phone image.
Apple just tells you know to use Time Machine or your normal backup utiliity to get prior versions. That sorta sucks if you just want to have N versions lying around ready to use.
Fortunately, you can just copy the folder in the directory
~/Library/Application Support/MobilSync/Backups/
to one of the same name with a timestamp suffix.
iTunes will then list the device and the date the backup was from
when you go to
iTunes / Preferences / Devices
So, you can do that.
It takes up a bunch of space since there is no proper sharing between
the images, but it is way better than having only one image.
XXX I've read (but not verified) that an iPhone backup may not backup t he actual content of your iPhone -- aka the music, photos, etc. It will only backup the iPhone settings, DBs, and Applications. To get the content backed up you need to have the above sycned to your mac (or whatever platform the iphone talks to). I have not verified this yet, ... be sure to sync your contents instead of potentially loosing them all.