I've run into the same issue, more or less, for the first time in the 3+ weeks I've owned the phone. This never happened up to this point, then I noticed earlier this evening that the phone shut itself down and would not turn on via holding power button down, etc. - a "ghost shut down", if you will. The only way to remedy this was a battery pull. Then, about three hours later (20 minutes ago or so), I noticed the phone was off, again - same situation, tried plugging into charger this time - would not charge - had to fix again with battery pull. Should I be concerned? I am four days from my 30 days since purchase, so I'm not sure whether or not I should bring it back for a replacement...
Can't remember if my phone's shutdown, but I think it has. I've experienced this same issue as well though. As stated, a battery pull fixes the issue, but I've had my Droid since the last week of November and only encountered this issue a few times in the last 2-3 weeks...