As if I were a teenager, yesterday I found an mp3 song that I hadn't heard in maybe ten years and a smile came to my face when I imagined that it could be played in my mobile when I need a loud tone (I am on of those who preffer having it silent all the time)
So I passed the MP3 directly to the mobile but it failed, the mobile can play it but can't be set as ringtone. Rapidly browsed my default existing tones and I saw some AAC files, and I thought to myself... Isn't iTunes importing CD's by default in AAC format? .... and YES
So if you want to set as ringtone one or several mp3 files, open iTunes and go to:
and select Import using: AAC codec
Then select all the files you want to convert and select the menu:
Advanced -> Convert selection to AAC
Now you can send the converted files to your mobile via bluetooth/whatever, save the file, and set it as ringtone :)
NOTE: That works for my old but useful Nokia 6630 with GPS and TomTom. I don't know much about which manufacturers are accepting AAC format, maybe your mobile is even accepting MP3 directly as ringtone.