You may have gotten an answer already, but I just converted a few of my iTunes songs to ringtones. My understanding is that the ringtone files should be around 15-20 seconds long - I think longer songs may not copy to the phone. What I did was the following:
1. Import a song into Audacity - it's a free cross-platform audio editor which will allow you to save songs in MP3 format (although you have to download the LAME encoder) - details at:
http://audacity.sourceforge.net/
2. Edit the song in Audacity to extract a 15-20 second sound clip - Audacity has either built-in or downloadable effects such as filtering, fade-in, fade-out, and many others. When you have your clip, export it to MP3 format (use something less than 192 KBps for sound quality - this isn't being played through your home stereo so you don't need optimum sound quality and you'll save on file space).
3. With your Mac and RAZR connected via bluetooth, hit the "Browse Device" option on the bluetooth menu, and navigate to the "audio" folder. Drag and drop your new file and wait for the transfer to complete.
4. You should now be able to select the new ringtone on your phone.
Good luck!
John
iMac G5 (PPC, 2.0 GHz) Mac OS X (10.4.6) Motorola RAZR V3