The spec MS are pushing for Windows 8 currently allows for the end-user to disable the "secure boot", which would let other OSs to boot, but there's no reason that couldn't go away in the future.
There are rumours the agreement with MS requires *all* products to conform to this. However, a manufacturer like Dell would be in a strong position to get that nixed for them. Dell sell a *lot* of hardware to corps who promptly put Linux on.
Wade.