Wirelessly posted (iPhone: Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_0 like Mac OS X; en-us) AppleWebKit/532.9 (KHTML, like Gecko) Version/4.0.5 Mobile/8A293 Safari/6531.22.7)
Seems browser cookie based to me. Should just be able to delete your cookies if you want to opt back in.
Since the OP didn't mention it, merely entering the URL above will automatically opt you out...there is no confirmation dialog.