Once again, this is what is being discussed here and what is currently unclear.
Worst case being the only key included is the one MS used to sign Windows 8, and even they will use another for Windows 9 (highly unlikely).
Best case is you can add your own key, preferably by setting a jumper or pressing a hardware button -- this is thought to be how it would be done in the real world. The problem here being that MS has as long history of giving discount licenses to OEMs who do things to make it harder for users to choose their OS (they've been convicted, this isn't speculation).
A middle ground would be, say, that a key is present for a CA who will sign Linux bootloaders -- problem there is that it will cost everyone who wants their kernel signed, since I doubt they'll provide a free service.
The developers working on the kernel, GRUB, or whatever could sign them, perhaps, the only problem there being that the GPL would have to be re-written or they'd have to give out the key used to sign them, thus defeating the object.
Of course, the manufacturers could just allow non signed bootloaders to run instead but, as stated above, MS will give them no incentive to do this and past performances point to the possibility of them actively discouraging it.