Bethesda acquired id software, so they are likely going to use id's latest engine. For those unfamiliar with the company (is this necessary?), the engine programmer of id, John Carmack, is a master of game engines. He created the engines for Wolfenstein, Doom, Quake, Quake 2, Quake 3, Doom 3, Quake 4. He always breaks new ground, and never fails to make extremely powerful game engines that are always moddable.
As far as the engine goes (if they are using Tech 5), we can expect some seriously massive, intricately detailed terrain that use something Carmack created called MegaTexture. This allows humongous terrain textures with dimensions of 128,000 x 128,000 (thats no typo) which is about 20GB of texture data.
Also, the most detailed characters ever in a game engine may emerge in TES:V Skyrim in the form of voxels, where game models are no longer stored in a mesh of triangles, but in volumes of tiny blocks. See the http://www.youtube.com/watch?v=VpEpAFGplnI
For convenience, here's the http://en.wikipedia.org/wiki/Id_tech_5
Again, Bethesda using Tech 5 is just speculation. Seriously though, why not? Bethesda isn't really known for the most rock solid engines, but John Carmack is! Cross your fingers!