Sounds like over heating. Anyway the problem with laptops is that if they don't get enough air they will overheat. Be glad it shuts itself down for protection. A friend of mine has no such functionalty on his laptop and it just heats up until the solder in the power connection melts - then he had to take it into HP to get it fixed. In fact they were going to charge him to get it fixed until I called them up for him and explained in legal terms why they were going to fix it for him for free.
I think the moral(s) of the story are:
As Prometheus mentioned, always stick it on a flat surface. I have a wooden board, so I can stick it on the board and stick it on the bed.
Be thankful it switches off before its insides are liquidised.
Never sit a laptop on your lap (apparently there are lots of cases where men have (very slowly) cooked their testicles because of the heat.
http://www.newscientist.com/article.ns?id=dn6777