You can't use an MP3 from your iTunes library as a ringtone, but you can make custom ringtones using iTunes, and your phone doesn't need to be jailbroken;

http://www.intomobile.com/2008/10/07...-itunes-8.html

Or use this;

http://www.efksoft.com/products/ipho...aker/index.htm

(disclaimer - I've not used it so can't comment on how good/shite it is/maybe)