The bug seems to be client based. I discovered this when on my server. I had attatched my horse to a leash, and then left the game to go to school. When I returned to play on the server it appeared not to be attatched, however, the rope is still wrapped around the fence.

This was on the latest development build of craftbukkit so it may have to do with that, but just incase, I'm uploading it to here. I'm sure EvilSeph will be able to find out.