What would happen to the Bitcoin network if all ISP's collectively block port 8333 at once? Will nodes still be able to find each other using today's official Bitcoin client?
The core developers could decide on a new port then get the word out that everyone should start using that port.  It's a very easy change in the bitcoin.conf file.  Mining pools, online wallets like blockchain.info and Coinbase, and "light" wallet providers like Electrum could lead the way.  (For example, once blockchain.info made the port change on their server, all their users would automatically have service again).  I think it would recover quite quickly.

The new port could also be shut down but it would turn into a game of "whack-a-mole".  The developers would probably come out with a new release that used some kind of strategy to dynamically search for a port to use to solve the whack-a-mole problem.

Alternatively, there is no reason they could not just switch to using port 80 for bitcoin.  The main disadvantage is it would make it a bit more complicated to run a web server and bitcoin node on the same machine.