http://www.youtube.com/user/OMFGcata#p/u/2/QhksNvUwaRc -A video of Jesse Cox exploring it with a group of about a dozen builders.
So, a group of people with too much free time decided to try to build Middle-Earth. They started in October. The scale ranges from 1:1 to 1:3 if I remember correctly (one of the constraints being the height limit).
They also have a website, minecraftmiddleearth.com.
According to who I assume is a high-ranking member of the community, it would literally take 6 hours to walk through the map. You could actually get lost in the Mines of Moria forever.
You can join them, though you have to have Ventrilo and be over 16 years old.