The usual failure mode for compressors seems to be for them to lock up.
Believe me you'll know if this is the case...either the belt will break or the engine will stall with the AC on.
And the bad thing about Ford's compressors is that when they do lock up, they usually distribute bits of crap ALL OVER the system. It's called "black death" (do a google search for "black death" ford compressor and use the quotes).
Brian
