Some people impacted by the issue note that they have been able to solve it by uninstalling certain third party apps, with Skype the most common culprit. Others have fixed the issue by resetting the device (go the Settings app, about module and choose "reset your phone").
Here's the statement from Microsoft:
We're continuing to investigate some reports of phones rebooting and have identified a cause with our partners. We are working to get an over-the-air update out in December.
With Windows Phone 8 Microsoft has introduced a system of OTA updates, replacing the previous PC-based update process via the Zune client. This makes it easier for Microsoft to deliver updates quickly to the entire install base of Windows Phone devices. Windows Phone 8 OTA updates can be divided into several categories: major updates, minor updates, and fixes. The first two of these will require operator approval, but the third, primairly aimed at delivering "hotfixes", such as the one described above for the rebooting issue, should not (for most operators, as always there will be some exceptions). In addition to updates from Microsoft, updates can also be issued by manufacturers. These will usually be for small updates and hardware optimisations.
The notification and download of updates is controlled through the phone update module of the Settings app. It can also be used to manually check for updates.
As an aside, we noted in our original article on Windows Phone 8 it is also going to be possible for end-users to have greater control over the delivery of updates:
Microsoft is also planning to roll out a program that will give early adopters access to software updates in advance of general availability. It's unclear exactly how this will work, but it may provide a way for users to be freed from operator limitations on software update deliveries.
Microsoft has yet to release any additional information on how this process will work, but it is likely to include a warning that by installing the update you will be opting out from operator technical support. We expect this process to apply primarily to major updates, rather than small hot-fix updates such as the rebooting issue described above.