Ever wanted to make a ringtone for your iPhone, but didn’t want to pay again for a song that you already own? Well, now you can…..Simply use Quicktime Pro (not the new dumbed-down Quicktime 7 player)
STEP 1 - Open your music source file and select the desired ringtone portion
STEP 2 - Crop to that selection and then "Save As iTunes Movie" with some name like MyZippyRingtone. QTPro will save it with the .m4v extension
STEP 3- Change the extension to .m4r and drag the file to iTunes, where it will be added to the library as a Ringtone
STEP 4 - Sync it to your iPhone and select it from the Settings screen
(Thanks Tom N. !)