Here's a link of how to do it:
http://macmost.com/make-your-own-iphone-ringtones.html
I haven't done it myself, but I've worked with a lot of audio files recently, and it seems like it'll work. (I'll be trying it soon ... My teen kids want to make some verbal ringtones for my new 3G S ;-)
One small issue, though ... these steps work best with an Apple computer (laptop, iMac, etc). Trying to do this with a PC will get pretty messy ...
Good luck!