One of the iPhone-Elite members posted a method for patching the MeCCA framework binary to allow for custom ringtones.
Source: http://iphone-elite google code page/wiki/Ringtones111
I followed the instructions and am just releasing a pre-patched MeCCA file for easier use. All credit goes to the iphone-elite team! I patched it using XVI32 (a hex editor)
1. Backup MeCCA file located at /System/Library/Frameworks/MeCCA.framework/ (I renamed mine to MeCCA.bak)
2. SCP the included MeCCA file to this location
3. Set permissions to 755 (I used WinSCP to do this)
4. Reboot iPhone
5. Copy your m4a ringtone to /Library/Ringtones
6. Change the extension of your m4a file from "name.m4a" to "name .m4r"
NOTE: THE SPACE BEFORE THE DOT IS IMPORTANT, IT'S WHAT THE PATCH LOOKS FOR
7. Go to Settings->Sounds and select your ringtone.