Just another data point...

I received an N800 as a gift on 12/12 and it's also exhibiting the power on/off problem.

None of the suggested tricks worked for me except to just wait and try it again. If I wait an hour or two after powering down, I have about a 50/50 chance of success.

I tried pulling the battery for different intervals and it didn't improve the success rate.

I managed to flash it. I didn't upgrade to OS2008. I just reflashed the current version thinking it might correct something, but no.

This unit was also purchased at buy.com. I'm planning to return it.