i have done this and it works fine with no
sms messaging or anything that can compromise you phone number:
1. take the mp3 and rename the extension .avi (same encoding without the video)
2. get the free 3g2 converter pazera and convert your avi to 3g2
3. load it to your sd card
4. ringtones can be selected from uploaded videos
easy as pie
edit:
1. you need to put the 3g2 file in the DCIM folder
2. videos over 512k cannot be loaded as ringers
and as for panzera: this free program rocks for uploading videos to your phone even full movies... nice