Well I am on day 2 of no shutdowns. The last things I did were as follows...
1. Installed the latest Intel Chipset Driver
2. Tightened down the CPU / GPU heatsink
3. Reseated the system memory
4. Updated to BIOS 2.03w
One of those seems to have fixed my shutdown problems. I honestly could not tell you which one is the divinitive fix, but I think it was the memory not making a proper connection to the motherboard and shorting, causing the system to shut off right away.
I would say if you are experiencing the random shutdown, try one or all of the above and see if it fixes it for you.
Also keep in mind, if Alienware were to sell 200,000 m5550's, and 100 people have the shutdown problem because of, lets say, improperly seated memory.. then that is less then 1% of all the systems sold. These guys are in a hurry to get these out the door to us, and when your puttig 100+ sticks of memory in a day the chances of getting a little lazy and just clicking em in real fast becomes easy to do. WE take great care when we do anything with OUR systems, those guys do the same things over and over to systems on an assembly line. Way different story