There is a nice writeup of how to add custom ringtones to the iPhone. MacOS X users only, for now, unfortunately (Update: Now available for windows).
Custom iPhone Ringtones
Custom ringtones are one of the biggest requests by people that want to modify their iPhone to make it unique. This page will teach you how to add your own files to the iPhone to use as a ringtone. The only thing preventing Windows users from doing this is the lack of someone to write the interface program.
Update 7/18: The iPhone wasn’t unlocked over the weekend, but there has been progress today. It’s now possible to use the iPhone with a variety of pre-paid plans that use AT&Ts network.
Have tried this about 15 times in 3 different Macs and just does not work,
The Ditto 6 error is preventing it to work!!!
Apple would be foolish not to allow third party ringtones, like at iphone ringtones