There have been people doing 100% submersion (excl hdd obv) sub-zero liquid cooling.
the test system was pretty old, all worked well down to about -50 or so, when it started going wierd. the thing that died was the electrolyte in capacitors freezing, (stops the cap working - and cause cause them to burst)
if you have a very new mobo and other components using soley solid-state caps then you should be fine for lower temps.
the biggest problem with this is: hot spots.
if you have a freezing cold board, with localised hotspots (for example on my abit AX8, above the pcie slots, there is almost nothing heat producing, bar a single FET that sits at about 85c) you will have a huge ammount of force and stress exerted at that point. it will almost without fail cause cracks in solder joins, traces or the entire pcb as the 2 heat up/cool down relatie to each-other. (submerging the entire mobo gives it a huge area to cool itself from, and its full of copper remember - but this mosfet even submerged like that has only its small surface and the solder joint to disipate heat from, and there will be little circulation of coolent here)
another consideration is ensuring you bring EVERYTHING down in temp together, for the same reasons.
moving parts that rely on lubricants will be the first to die in extreme cold, eg fans and hdd motors. this isnt a problem usually, if they are already running - but will be a huge issue when starting them up.
i would fully expect an off the shelf pc to work just fine in the arctic (protected from water, ofc) so long as it HASNT got uber zooty case cooling pulling a thousand cfm thru every second. and the hdd's are sat all together in a nice toasty cage with no ventilation. this is all on the condition you switch it on someplace warmish, then stick it outside.
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the disadvantage with pc-in-a-fridge ideas is - these are devices that expect to have no heat producing items in them, they expect warm things to be put in, that must have het extracted. nothing actually making heat. putting a pc in a fridge usualy causes 1 of 2 things: - pc melts because the fridge cant nearly move the heat output from inside its insulated compatment out fast enough, fridge melts because it was never designed to run full power for more than a few minutes. - feel how hot the compressor is on your freezer right now, it will most likely burn you, and thats when its only running 10 mins an hour, say.
