Grub2 will apparently operate much the way it would if you had set a supervisory password in your configuration.
Once the kernel is booted, it will also detect that it is in Secure Boot mode, which will cause several things to be true: it will validate the boot command line to only allow certain kernel settings, it will check loaded modules for signatures and refuse to load them if they are unsigned, and it will refuse any operations from userland which cause userland-defined DMA, the project notes.
Arriving in Late October
Windows 8's Oct. 26 arrival date is still a long while off, of course--as is Fedora 18's November release date.
Still, Linux distributions have much to figure out to ensure that users will be able to run their favorite Linux flavor on the hardware of their choice.
It's going to be interesting to watch what the other distributions come up with. Meanwhile, my favorite part may well be the Fedora project's contingency plan: Gin. We may do that anyway.