OK, the secret's out! I had suspected this, but now I know for sure:
You can pick each side's intelligence:
Human (you)
Computer (normal default)
Computer Overmind
Computer Neutral
I was testing a new level's terrain. I had a small enemy base built, with some bullpups, and a few guards of my own to explore with. No triggers, no unit groups, nothing. I did select Overmind AI.
Imagine my surprise to see the base building Units! It appeared to be random, but it was building a good assortment. It didn't attack my guards until I got too close, though. So, I added a few structures of my own and recompiled. Within a few minutes, I was being attacked by the enemy!
You can still command the units, but they are smarter, and more aggressive. Not quite a skirmish mode, but this may save some people from getting too bogged down in the programming.
Hope I'm not the last to discover this
