iToner from Ambrosia Software is a Mac utility app that allows you to add your own ringtones to the iPhone. This will allow you to make custom ringtones for your iPhone from your mp3 files. It easily transfers MP3 or AAC audio files to your iPhone using drag and drop, and sync. See some screenshots of the software here. The app costs $15. If you are willing to unlock your iPhone you can also try Song Sender which is a free utility mod.
Here is an iToner video demo:
Try it here
Now you can turn any AAC file into an iPhone ringtone in 3 easy steps. Make sure you have iTunes 7.4.1
Next time your connect your iPhone and go to the Ringtones tab in iTunes you will see your .m4r files that you can sync as ringtones.
There is now a new app for the Mac called iRing which automates the above steps.