Originally Posted by Youngbinks
I think iPhoneRingtoneMaker allows you to make ringtones using songs purchased through iTunes whereas GarageBand does not. I'm not 100% certain though as I don't use iRM.
Might be a little more trouble, but what I do is burn the song to a cd-rw. After that, you can import it into GarageBand and make your ringtone. Of course now, with so much being dram free in iTunes, it is becoming less of a problem.