Secure Boot isn't a Windows 8 feature, it's an EFI feature that Windows 8 will require. the concern being that Microsoft will convince manufacturers to lock out non-Windows OS's from being installed.
I don't think it is so much Microsoft convincing OEMs to lock out other OSes, it's more like MS relying on OEMs being lazy and not putting the option to load your own keys into the BIOS.
Maybe, maybe not. Microsoft DOES have a history of strong-arming OEMs into being Windows-only. If they get lazy it'll only be because they don't see Linux as a threat anymore.
I think that distinction is pretty important.