I have an Alltel Motorola Razr V3A with S/W version 38.0_X_02.21.02R
I have created and applied numerous mp3 ringtones to this phone, and have never had any kind of problem.
Yesterday when I created a new ringtone and applied it to the phone it would not allow the new mp3 to be played back. When I browse to the file and press play, the progress indicator starts to travel from left to right and then the phone simply says "stopped".
I can bluetooth the ringtone from my phone to my son's phone (a rokr) and it will work just fine.
All of my old home made ringtones continue to work, even if I delete them then re-install them from my laptop.