Hacker par excellence Cleverboy, aka Dudley, has pretty much figured out what makes ringtones ringtones as far as iTunes is concerned. He discovered an atom in the m4a metadata that identifies the file's role. He downloaded a copy of AtomicParsley, the command-line metadata editor, and after a bit of hacking discovered that setting the "stik" metadata to 14 turned any aac file into a ringtone. He writes, "The file immediately appeared in my ringtone list, and after syncing, it appeared on my phone without one iota of complaint. --NONE".
The command in question is as follows:
AtomicParsley mmbop.m4a --stik value=14