Reading around the intarwebz this morning, I came across this link, detailing how a player discovered Vvardenfell and Tamriel past the set borders of the Skyrim world.
I'm not sure I agree with his idea that the extra lands are there for modders. Bethsoft has really half-assed their mod support lately; despite, as he correctly notes, their heavy reliance on the mod community to sell their crappily coded games. Landmasses that large, we're talking dozens; maybe hundreds of hours of heightmap editing and such -- I just can't see them doing it 'cause they love us so much.
I would say it was a holdover from Oblivion -- that like FNV, they had simply modified an older iteration of the engine and been too lazy to clean the previous game's content out -- but I don't recall Oblivion having Morrowind content, so that theory doesn't hold up.
I think my vote has to go to building blocks for future planned DLC. Though I must confess, adding Tamriel and Vvardenfell seems a bit... expansive for DLC.
It's a shame the CS isn't out; I'm beginning to get a serious wonder about just what-all's hiding in that esm file...