Well this is really neat. As you've probably heard, Skyrim is a massive game. It's even bigger than you think though. A PC player, after discovering a gate-like structure in the southeastern portion of the map, turned on a "no clip" mode to explore beyond the Bethesda-set boundaries. This led to a surprising (and totally awesome) discovery.
It turns out that Morrowind and Cyrodiil, the environments featured in the two previous Elder Scrolls games, are built into Skyrim. There's not much to be found other than low-resolution landscapes, but Bethesda put in enough detail for key features like Morrowind's Red Mountain and Cyrodiil's White Gold Tower to be visible.
This raises some intriguing possibilities. It's unlikely that these landmasses exist solely for the purpose of making Skyrim feel like it's part of a larger world. There are regions, particularly Cyrodiil, recreated with a fair amount of detail that would never be visible from Skyrim's highest peaks.
The player who discovered the content puts forth two seemingly reasonable possibilities on Tumblr (via Destructoid). One: Bethesda used the placeholder environments as a reference point for its modding commuity to work from. Two: these temp regions are placeholders for future DLC.