Windows Phone Blog detailed the process for getting custom ringtones on WP7 in Mango
Ringing in Mango
Seems ridiculous to me. All they've said so far is that the file has to be on your PC, you use Zune to change the genre to "RINGTONE" and then sync it to the phone. If this is the only way to do I think MS dropped the ball on this on.
Why shouldn't I be able to download a sound from the web directly to the device and then assign it!?
I hope this gets cleared up by the time Mango launches.