its fiddly actually, but you can do it within iTunes. Writing this from memory, so perhaps it's a little out... For whatever reason, Apple will only take maximum 30 second audio files as ring tones, so step 1 is to delete the hell out of your MP3 by using iTunes ability to pick a start point and an end point for playing songs...ie, start point just before "I" and end point just after "was made for lovin you baby". Then right click on the song in itunes and select covert to AAC. The conversion will skip all the part that is selected to not play, and you will have a file under 30 secs. (side note - at this point, go back to the original MP3 and reset your start/end points to 0.00 and the end of the song). Now you just import your new AAC file into the ringtones thingy in iTunes, select to sync ringtones when you sync the phone, and away you go.
