We've been looking for a way to allow our clients to add ringtones to their site. There are a bunch of cheap hacks, but the user experience isn't so hot. Just too many hoops to jump through for the average Joe.
After much web searching, we finally made contact with 3G Upload - these guys make it easy and affordable for us to add ringtones to our sites. Basically we provide them with MP3 files and they tweak them to fit a variety of phones, both MP3 and Polyphonic MIDI as well.
We then use their tools to build a download tool into our website design. Not bad, we hope to use them soon.