Tor relays/bridges can detect when they are being filtered by SSL cert
The blocking events from Iran in January 2011 and September 2011, and from Syria in mid/late 2011, share a common theme: the client establishes a TCP connection, sends an ssl client hello, and then goes silent.
In theory Tor relays could track if there's a sudden spike in connections like that from a given country, and report it.
I've put this ticket at 'minor' priority since I think the right plan for us (given all the actual dev tasks we need to do) is to hope that some nice volunteer does it.