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.