With the most recent patch changes they added deserter to arenas. This helped prevent a few exploits. The obvious way around this is to get in combat before leaving the arena (allowing you to still tank rating, etc.) as this was stated in the patch notes.
Now the real exploit comes in how this affects battlegrounds. You can currently /afk out of a BG while in combat (haven't tested it with getting in combat then leaving combat but I feel like this should work as well) and not get the deserter debuff. This is a great way to leave losing BGs to get in a better one.