Not just Linux...
The way I understand it, it could also effect:
* Volume license users with downgrade rights - Which versions of Windows have signed bootloaders? Do Vista and 7?
* System imagine tools similar to Ghost, Acronis, etc.
* System tools, like hard disk tests, RAM tests, etc. Especially non-vendor specific ones.
* Offline virus scanners that boot off a CD.
Now, any and all of these could end up with signed bootloaders, but doesn't the system's UEFI need to know to trust each signature? If they only trust MS, all the system tools are still screwed. How many can be trusted while still keeping this whole thing secure?
To me, it seems like a big pain in the arse with extremely small results. Rootkits have to be one of the rarer infections, I'd have thought.