I don't think this is a problem with the phone. There seems to be a lot of people that are using old backups and restoring them to the D3 which can cause problems. In addition to that, there are also people that are taking their microSD cards directly from their old phones and putting it right into the D3. Again, more problems.
Yes, some problems are occurring due to battery issues but I find it hard to believe this is a widespread hardware issue or that it resides in the original software.
From a personal opinion, I have not experience ANY reboots at all for the week that I've owned my D3 so far. I did not restore apps from back up and I formatted my microSD card before transferring it from my D1 to the D3. I know I only speak as one person but so far I've had 0 issues.