same problem here, but when i updated from 1.1.1 to 1.1.2 and when my phone was still not activated i tried *#301# and when it called it played my custom ringtone i had from 1.1.1 and now it doesnt
i learned that, you have to keep the exact file you send to your ringtone file in your ipod part of your phone, if you delete the song or change it in anyway it wont work as a ringtone. also, restarting your phone might help as well
Ive also had this happen a few times. I simply selected a stock ringtone, exited out of the ringtones screen so it shows the stock tone as my default. Then went right back in and selected the custom tone and it works after that. Ive had to do this everytime I send a new ringtone there.