Garageband 08 has it built in now...just drag a song to a new track.
Change the cycle length. Click share->send to itunes as a ringtone.
Now I have what I consider to be the ultimate ringtone ever.
Pink Floyd - Time -- The part with all the clocks going off :)
--
Look at the shoes your filling
Look at the blood we're spilling
Look at the world we're killing
The way we've always done before
Look in the doubt we've wallowed
Look at the leaders we've followed
Look at the lies we've swallowed
And I don't want to hear no more
