Q:

Building a waterproof case

I'm tempted to use the Pi for controlling an autonomous robot that could be subjected to harsh conditions - mud, water, the lot. ( It's theoretical at this point, but the concept remains the same!) 
Because of this I'd either like to build or buy (relatively cheaply) a completely waterproof case, preferably one rated for submersion up to 1m to be safe. 
What would be the best way of obtaining such a case, and how could I safely route cables through it whilst still maintaining its waterproofing? 

A:

Many cameras and video cameras have waterproof cases available, most of these have buttons on the outside connected to mechanisms for pressing buttons on the actual contained device. 
As other answers have mentioned, waterproof cables with underwater connectors do exist, the problem you might face however is that the off the shelf waterproof casings probably don't have holes for you to thread these through.   Depending on what you wanted you can by watertight boxes with connectors already attached or make your own holes and make sure you properly seal them. 
You could also consider enclosing the power for the RPi and its power supply inside a case and have it communicate with the other robotic components via wifi/bluetooth. 
Example of the kinds of wire seals and boxes you might use: http://www.fuertecases.com/wireseals.html

A:

There are cheap cases on the market that are waterproof.  They use rubber isolation in the caps for the cables. 
What I am more concerned about is the cooling for the processor core.  You can use a radiator which will scatter the heat. 

A:

For connectors, try these:
http://www.samtec.com/panelio/IP68_Sealed_Systems.aspx
Not sure how deep you could go...

