I use the Blackberry Ringtone Generator, same idea as the above. BRG is extremely simple to use; point at file, move bars around that mark stop and start times, generate ringtone. Done.
Edit: unlike Audicity, it's not free though.
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-Klotar
Supervising Manager of the Department of Redundancy Department.
BlackBerry Certified Support Specialist
I use MakeOwnRingtone.com for the same purpose.
I really like the free tool. It's a big help if you want to create your own ringtones out of mp3 files quickly.