Blogger Martin Backschat has more or less translated a German article that takes a stab at trying to figure out what is going on with this random MacBook shutdown issue. One only needs to check out MacBook Random Shutdown.com or comb the archives of digg and Apple discussion forums to grasp how many are plagued by this dark game of workflow Russian roulette, but we have yet to hear even a peep from Apple as to what the problem is, and how they're going to fix it.
Martin says the theory centers on a cable that runs between the heat sensor and the CPU's heat sink being too short. The heat sink expands when operating the MacBook, which causes it to contact the heat sensor's cable and melt its insulation - hence, a short circuit and a shutdown. Once the MacBook is no longer running, the heat sink cools down and contracts during the process, breaking the short circuit and allowing the machine to boot again (this expansion and contraction can happen pretty quickly, so it would make sense if your machine can boot almost immediately after shutdown).
It's a twisted game these components play with each other for sure, and we're all hoping for some kind of a statement or - ideally - a solution from Apple soon. The more widespread and publicized this issue becomes, the farther away these Macs will get from their 'it just works' reputation. Something tells us 'it just works - until it decides not to and randomly shut down, blowing away all your work' wouldn't roll off Justin Long's tongue nearly as well.