Using IP connected arduinos to trigger cameras
TCP/IP on Arduinos isn't exactly the best choice for anything real-time.  Ethernet timing isn't deterministic, so regardless of any method that attempts to remove jitter or compensate for latency, it will never be spot on.

You mention "Arduino -> network -> Arduino" as your desired topology, in which case I would throw away the idea of using the (very expensive) Ethernet boards and use something designed for embedded systems, like RS-485.  As you have total network control, your master module could send a 'shutter' message for every slave to pick up and act upon after message completion.  They would all receive it at the exact same time (+/- 1 ns/foot), so jitter and lag would be negligible for any camera application.

Here's a shield that has RS-485 on it, but it's just a single chip (e.g. MAX485 &amp; friends) that you could breadboard.



Edit:

How far apart are these cameras?  If they're only ~10 meters apart and you want a (light?) sensor that goes to each one, just wire up a sensor+shutter+power cable and run them to the same Arduino.  It should only be 4-6 wires; buy some good connectors and it should be painless to tear-down and set-up.  You might be able to get away with significantly longer cables even, depending on the sensor, etc.