There's a gate into Morrowind apparently:
http://media.tumblr.com/tumblr_lv1l1a6c7h1qb5ru9.png
And if you use noclip to go through it and go far enough here's Vvardenfell:
http://media.tumblr.com/tumblr_lv1m69mbnR1qb5ru9.png
Cyrodill is there as well, here's an overhead compared to a map from Oblivion:
http://media.tumblr.com/tumblr_lv1mpvuMXL1qb5ru9.png
Full blog post is http://ppsh-41.tumblr.com/post/13145143504/entire-tamriel-landmass-built-into-skyrim. The guy hasn't fully explored it yet and as a PS3 player I can't, but I figured someone here might want to.