Haha, slightly upgraded Gamebryo, beautiful :rofl:
If you look closely at the Skyrim videos and screens that have been released so far then you can easily see the similarities between the new Creation Engine and the Gamebryo engine (The modified version that BGS used).
My guess is that the BGS programmers used Gamebryo as a model for the new engine so they could work in a familiar environment. I'm not saying that Creation inherited the flaws in Gamebryo but there are definitely similarities.
It makes sense when you think about it. It's extremely difficult to change engines in the middle of the development cycle and since they had a lot of experience with Gamebryo and it's predecessor (NetImmerse, which they used for Morrowind) so they probably thought it would be a lot easier to create something they could easily adapt to; something similar.
This is all just guesswork of course.