I'm not talking about waterproofing for underwater operation.  I'm talking about waterproofing for "oops, one of the tablets fell into the pool again"

I'm a bit leery about Titanium because I understand its strength is in its flexibility, which causes problems when talking about strength of the internals, which is not.

I've been thinking about the Aluminum in the MBP.  My 15" powerbook survived a 8 foot drop onto concrete with little more than some dents and a scratch on the screen.  If I planned on the same grade Aluminum, with about 3mm thickness (I think that the MBP is 1mm) I should be able to get a considerably stronger case without so much of the steel weight.

Also, after looking at the internals of the 17" MBP, I am thinking that I would be able to make the top out of a passive copper heat sink at the top of the unit, then utilize the existing cooling system to circulate the air internally.  Doing so, would effectively seal the electronics, but allow for heat transfer to the outer air.

I wonder what the smallest enclosed airspace Apple would want a MBP to be run in without it overheating.  Given that I am planning on making a metal enclosure that is 16"x11"x2", there is much more air in the unit to transfer the heat out of the heat sinks, and transfer it into the enclosure.