I don't know about Windows 7 or Server 2008, but I do have experience of Windows 8 CP.
One of the advanced reboot options is to disable driver signing enforcement for the next boot. You can then install unsigned drivers by clicking through a scary warning as in previous versions of Windows. Once that driver is installed, you can reboot in normal mode and continue using it. I'm not certain if there's a boot flag that can be set *permanently* to keep enforcement disabled, but in practice that's only going to be a problem if you need to install unsigned drivers on a frequent basis, and to be honest I can't really fault MS for not considering that a high-priority use case.
Since the advanced reboot options menu is entirely new to Windows 8, I doubt it is a left-over from old versions that they're planning to remove in the final release; more likely that's how it will work in RTM.