Until someone actualy buys one and tests/measures it everything is speculation but my guess is that the mirror will be identical as why would Canon change it ?
The mirror should have a decent chance at being slightly different.
There are also already enough people in Asia with this camera in their hands for at least one poor Rokkor owner to be or not to be slamming his or her 5D mk II mirror into rear glass retainer rings.
The mirror/mirror box is supposed to be very similar to the original 5D. The only change I have read is that Canon has added clips in the attachment of the mirror so its not just held by adhesive as in the original 5D.