This is something that actually concerns me. I know that I should not be complaining, but part of the fun of H2 was FINDING the skulls in the first place. So by having those sitting there it makes them not a challenge. Maybe ,since they were on Normal difficulty in the playthrough, there are different skulls. Or it was just a teaser to show that skulls are indeed in the game. They would have had to know where to go to find the skull if it were hidden, so 343i decided the best way to "re" start the skull hunt was to shows us one at E3. I will be VERY disappointed if they are not original in placement.
I distinctly remember having 2 profiles saved for IWHBYD. One was with the infinite active camo glitch, and was one to try it legit. So many hours put in to getting that skull legit. (t'was a slight Legendary Noob)
I think what they will do is allow you to use them whenever, i.e. you don't have to find them to activate them. However, I would almost guarantee they use the achievements as motivation to actually find them within the game. Similar to H3 achievements. IWHBYD should be at least 75 gamerscore if not more!