I think the Enderman should only spawn near Obsidian, as it kinda makes sence, they're all black, like Obsidian...
It'd be so awesome if they specifically targeted Obsidian to move about, and if they had enough, perhaps they could make portals! (wait, that could prove deadly)
this is just my speculation, but as noth said, the best speculations will get implemented:
Enderman love to move Obsidian! As this allows them to spawn more Endermen to create an army! They like putting Obsidian in obscure places like:
-1 block below ground, so it's undetectable
-In the middle of 1x3 hallways, to stop people moving past, while still spawning more enderman
-they also like building Portals, but an Enderman must sacrifice himself in order to activate the portal
-Endermen are good friends with Ghasts, and will often team up with them, either by riding them, or attacking with them, but not attacking the Ghast if hit by a fireball...
-Endermen also enjoy tearing apart your base, allowing other mobs to get in and do their dirty work
^If someone can put that in a Minecraft Forum, I'd love to see something like that implemented. I think the Enderman should be an intelligent mob