Well, official word on this item just came in..
If you go to the Bungie store, they've now got a t-shirt there that has a picture of the Mjolnir armour being used in Halo 3, and it says: "Mark VI" under it.
So although there were SOME changes made, their weren't enough to change the Mark #..
uh... Unless the shirt is just inaccurate?
Also.. there's a new theme for the xbox 360 that's availible for free when you buy the Halo 3 faceplate, and it's been decribed as saying: "showing all three variations of Mjolnir Armour."
Unless there's some goofs, I think there's supposed to be 5 versions..
Halo 1, Halo 2, Halo 3, Halo Wars (older version of armour), and the one on the "Ghost of Onyx" book cover (assuming that's not just inaccurate artwork, or possibly not a Spartan, despite how similar the rest of the armour is).
MAYBE you could also suggest that it should include the different armour that was used during the creation of the original Halo game when it was still a 3rd person shooter. (as shown on the 2nd DVD that came with Halo 2).
I dunno'.. seems like a goof in the product description, but possibly it's calling the Halo 3 and the Halo 2 armour the same thing? I dunno.