That would be a digital shutter, aka turning the sensor on and off, like on the original 1D, wouldn't it? The old Nikon d50 is the same
Interesting, I didn't know about the 1d method - why don't they do that anymore to gain higher x-sync?
As for the 6d & 1/8000, I have no idea how it's implemented, just picked it up while reading through the ml 6d dev thread - you have to look or ask over there.
Sorry, the Nikon d70 and d40 do this, as well as the original 1D. The actual shutter still can only physically hit the sync speed (1/250 or whatever) but then the electronic shutter takes over at higher speeds. No idea why this is not a standard feature on all DSLRs. Maybe only CCD sensors can do this?
Read about the Magic Chip Cameras on Strobist