Well, I wouldn't claim to have a "god solution", but I do have some good ones ...
There is a big challenge here for ~1GHz CPU systems. Sticking a regular MB in a case generates many heat problems - if you vent the case, sure you can get the heat out, but lets moisture in - only a few climates are favourable for that.
We spent a lot of time and effort on this problem, and came up with a "mini range" of solutions which aspire to be true telecom grade outdoor routers. In all of them, the CPU/cooling system/case become an integral assembly.
Sadly, none of them are cheap, so anyone on a tight budget, look elsewhere.
In our laser systems, we use peltier coolers to protect otherwise-fragile lasers from extreme temperatures - which is excellent, but that's not exactly the same as removing 20W from a Pentium M (and 100W Pentium 4 - forget it!). Getting ~20W from the case is a different game, and a mix of active/passive solutions seem to be required.
Do send me a mail offline if you want to discuss. Put 'stephen.patrick' instead of 'info' in the address below -