I'm experiencing the same problem, also after installing the SMC Firmware update, on a white 2Ghz. It cannot be due to overheating, because it useually happens just after it wakes up, when it is cold, something my hands are able to verify.
I also have to try several times to get it back on. I found that pressing the power button until the small white light on the lower right corner of the computer starts blinking helps. When I release the button it beeps (like a PC BIOS being reset) and starts fine, and does not power off for a random amount of time, useually days. I guess the low frequency makes it hard for Apple to reproduce the problem. However it sucks big time. I've yet to experience serious data loss, but if I start using the Macbook for something important (still have my trusted PC for that) I guess it's just a matter of time with the current behavior.