You probably already found your answer, but my brother-in-law and I just figured this out the other night. We initially looked on this discussion board and found nothing, so I'll attempt to paste our solution on several boards.
The steps may look long, however, after you do this once it's so easy...
(1) Find the song you want as your ringtone and drag it into GarageBand. (If the song is a purchaed iTunes song it will be protected and this won't be possible. There is various software to remove the protection, but I have yet to use it).
(2) Cut the song down to a 10 - 45 second clip within GarageBand.
(3) Send the song back to iTunes (using the Share menu). The song will then most likely show up in a new playlist with your name as the artist.
(4) Convert the song to a mp3. You will most likely need to first go into iTunes preferences and select the Advanced setting. Then change the import (yes import even though you'll export) option to mp3. Then go under the Advanced menu and select convert to mp3. Now you'll have a duplicate song, but this one will be an mp3. Use Info (Apple "I") to verify which is which.
(5) Drag your new mp3 10 - 45 second mp3 to your desktop.
Now you'll send your mp3 mobile phone via bluetooth.
(6) Go into System Preferences and select Bluetooth. Then go into your phone and setup your bluetooth (if you haven't done this before it can be tricky because there are so many different phone types - however you'll basically need to go into your phone setting and send your signal to your Mac). The process is very similiar to setting up a bluetooth keyboard or mouse. Follow the onscreen prompts and you'll likely be asked to enter a computer generated number into your phone to verify you want to connect.
(7) Once connected you should then go into File Sharing and turn it on (within the Bluetooth menu).
(8) You're almost done. Now open Bluetooth File Exchange (within Applications - Utilities) and select send a file. Select the mp3 file (or photo or video) on your desktop and send it. Lastly, you'll most likely have to Accept the incoming file on your phone.
(9) Once the transfer is complete (1 - 3 minutes) you should have a new self-created ringtone!
Let me know if this worked!
