Hack The iPhone has posted an in-depth walkthrough on how to get custom ringtones working on your iPhone.
It requires quite a bit of hackery and, if you’re not careful, could very well kill off your iPhone altogether.
I honestly don’t think I’d want to risk that considering it costs a small fortune to purchase an iPhone in the first place.
If anything, it’s a pretty good read just to learn more about the inner workings of of the iPhone (atleast from a software standpoint).
The usual disclaimers apply. It’s your own fault if you break it. :)