Guys,
Same problem. One fine morning, the netbook just started shutting itself off after 20-30s.
It appears that the previous night, it had fallen down on the carpet and for some reason the fan had stopped working. I realized it as I couldn't hear the hissing sound when I booted.
After trying several things and scouting on the net for answers, I gently tapped the netbook around where the fan is and booted again. This time, I could hear the fan hiss. The netbook works just like before now!
Bottomline: It can be the fan problem. Hope this helps.