I think that they are robots, mwhahaha!!
Wouldn't it be kool at some point in the game, you can take over their control center and turn them all against their maker!
Hmmm, that has some interesting gameplay possibilities.
Er, we'll see, I'm pretty sure that whatever they are, it'll be...
Considering relative capabilities, I'd say Doom 3's engine and Source are about equal.
Source doesn't use full dynamic lighting to the extent of Doom3s
Doom3 doesn't use Shader effects to the extent of Source and so on.