, take a look at this DIY solution. An engineer from Germany took his own MacBook apart and seems to have found the problem.
The cause? The heat sensor cable that sits next to the CPU’s heat sink. As the heat sink expands during use, the cable comes into contact with the sink and gets its insulation melted, thus shorting the cable and causing the MacBook to turn off. After a while, the heat sink cools, shrinks, and breaks the short, which means your MacBook is fine to turn back on.
A solution is to bust open your case and place some insulation tape around the affected areas of the sensor cable. Once you do that—unless the tape melts also—there should be no problems with sudden shutdowns. Though this procedure may or may not void your warranty. You may want to just wait until Apple acknowledges and fixes this for you.
Random Macbook Shutdowns. Solved. At Last. Hopefully. [Maba via Digg]