I have heard via twitter that Bethesda's community manager, Nick Breckon, announced "We can now confirm that the TES V: Skyrim engine is all-new. And it looks fantastic." it seems that they will not be using the Gamebryo engine this time around, most likely due to the many reported bugs and glitches on their latest game Fallout: New Vegas. It is unknown as of now what engine they will be using, but it was most likely made with bug-elimination in mind.
as an answer to whether or not it was created internally he also said this: "It's a new graphics/gameplay engine built internally. We'll have more details down the road."