The routine is simple:
1. Use Quicktime Pro to export the .mp3 ringtone to iPhone format using the "Movie to iPhone" preset
2. Rename the resulting file's extension from .m4v to .m4r (you can do this during the export)
3. Drop the file into ITune's ringtone list (the PC's list, not the phone's)
4. Plug the phone into the PC and Sync ringtones
These instructions are for Quicktime and iTunes on a PC, but I expect they are similar on a Mac.
If your ringtones are in midi format, you need to export them (using Quicktime Pro again) to AIF format and then to m4r
In other nastiness, I haven't figured out yet what the difference is between ringtones and the completely different list of tones offered to notify of new text messages. Seems silly to keep those separate. And annoying.
In other news, the iPhone does not get a reliable signal at my desk, does not support voice dialing, and the keyboard is total suckitude. I am hovering on the edge of returning it and going back to the Motorola Z9 and Palm TX combo.