ACU rushes are really annoying. How can I counter them?
I agree with RCIX's point about the radar, and possibly some aircraft/tanks on patrol for spotting.
However, I find that this is often like a "queen swap" in chess; in the early game you can usually only defend against it by sacrificing your own ACU. Although, if your ACU has enough research on it it may well survive taking out their ACU before it gets to your base.
You really should start with building radar so you can see if they're ACU rushing. From there, if they are, drop everything and start building some point defense. If you have factories, then halt them and order the closest one to put up a factory shield, then build under that. 3 PDs will counter an unupgraded ACU and 5-7 will counter an upgraded one. If they get overcharge, your best option is to get shields and deploy several of them in your base.
My suggestion is that if you find an ACU rush from a distance, deploy a factory away from your base at a choke point, then quickly deploy a shield generator from an add-on. Add PDs (as many as possible) underneath the generator, and then, if you have some time, build some units for chasing your opponent if they run. Make sure you use a couple engineers for this, not the ACU, or else you're making the opponent's job easier. By the way, if you didn't know, engineers can assist a factory to build units/add-ons faster. Hope that helped!