This was mentioned in another thread, I can't quite remember where, but if you'd notice, maps aren't the tile-based system you see in games like Warcraft 3, etc.  They're each custom designed and rendered, and not as modular as they are in those games, so that's where that stands.

That being said, a lot of the code in the actual game is pretty open-source.  If you know much about programming, and were so inclined, you could pretty easily go in and fiddle with the code, which is something people have already done, though it's not supported online, since you can change pretty much anything, from UI things to AI to the way the units behave.  However, I don't THINK you can actually play with those mods in-game.  Somebody said they were disabled after the beta was over, and I tested earlier today to see if I could just screw around with some of the units, and DG had none of it.  The game still ran as normal.  So maybe I wasn't delving deep enough into the code, but so far modding isn't supported, but probably will be in some capacity in the future.