You need to have Ringo ($20) to make this work, but it's pretty cool.

A program called SoundStudio came loaded on my iBook, I guess everyone has it... anyway, under File -> Import with Quicktime, open whichever mp3 you want to turn into a ringtone.

Once its open, play the song until you find the portion you want. The reason for all of this extra work is that most songs don't begin with the part that you want to play as a ringtone immediately... so you have to edit them and crop out the rest... i used a specific part of the guitar solo from "Blue Sky" by the allman brothers for one of mine.

anyway, highlight the portion you want, then edit -> cut. Open a new blank file and paste in the portion. The next step is to save it... Sound studio won't let you save as mp3 I don't think, but name the file and remember where you put it.

Then Open iTunes and File -> add to Library. find the song you just made. Next make sure that your importing preferences are set to the mp3 encoder (you can switch this back if you prefer an other for actually importing CDs). Right click (new to macs and will never go back to pc, bu i don't know how the hell you do this with a one button mouse) the song and "Convert selection to mp3".

Once this is done, right-click it again and select "Show song File". this should bring it up in the Finder.

Hopefully you have that "Send to Handheld" applet in your Dock. If you do drag your mp3 you just made (not the aiff) onto it. If not, open a new finder window and look in Palm->applications.

Send the file to the SD card (you need this too).

Use Ringo to assign the song to whoever you want... have fun.