We don't know when the game will unlock on Release Day, nor why Civ5 was activated when it was. However, two different games, two different developers and two different publishers so any similarity between the two games is coincidental.
That's not really true. Reps from 2K made if very clear on Civ5's official forums not only that the game wouldn't unlock for anyone until Steam unlocked it, but made clear the time, and asked to be notified about any midnight launches because they didn't want them to happen.
And right now, Steam has a countdown time for Fallout:NV (as they always do for games you can preorder) that points to the time I mentioned in my first post.
It just seems to me that this is something that needs to be clarified, because people who buy the PC version at a GameStop midnight opening are going to be posting here and be upset if they can't play until sometime the next day.