I have MP3 ringtones that I created.
They loop fine with all the older, Windows Mobile, phones that I've had.
On the 8X, though, it plays once, pauses, and plays again (not a seamless loop).
It's quite annoying.
I'm not sure if it's specific to the 8X, or not (I'm the only person I know with a Windows Phone).
Obviously, I could create all new ringtones, but that's kind of lame.