That's the million dollar question
Because the second you can figure that one out, is the second you can make all of your ringtones look like purchased ringtones and then the iPhone will never know the difference.
I have a gut feeling that since all Ringtones come from DRM'd iTunes Music, that there's DRM in the ringtone and that the iPhone will only play DRM'd ringtones. And until someone can come up with a way of reverse engineering and applying Apple's DRM at will....it's going to be a long time before we see custom ringtones.
BUT as soon as we hack this ringtone issue, Apple will be forced to respond by issuing yet another iTunes and Firmware update. And the only way they can get people to upgrade is if they add amazing features that people want. So as soon as Ambrosia is done upgrading iToner to 1.0.3 maybe we'll get right click support :-D