Trijicon is putting a small red dot on top of the ACOG these days. And the obligitory bible verses Smiley what a poo storm.....
Funny story:
A good friend of mine I worked with up here in Canada before he moved to the USA to be the Military/LE rep for Knights Armament, Kevin Boland, came up with the idea to glue a homemade mount for one of our Doctor Optic small red dot's to the top of his Leupold M3LR Optic on his precision rifle, and on top of the Acog NSN on our carbines. This was back in 2000 or 2001. After a huge service rifle shoot that our team did at Fort Lewis back then, everybody had a lot of laughs at the idea, but it didn't take long for some guys to start writing about the idea with some pictures of our rifles that were equipped with this "back up close range dot" system. The idea spread slowly at first, but took off like wildfire a number of years later.
Fast forward to today, there are dozens of companies that make a dedicated mount for this setup, which differs little from the rigged up version we used 12 years ago.
Also, I have to agree with a lot of what VonMessa says regarding Iron sights. I know that in addition to my company, a lot of the other major PMC groups do much of their testing with new applicants with ONLY iron sights as a way to separate the wheat from the chaff. Triple Canopy for instance did all of their courses of fires over their 3 day testing rigamarole for new applicants with Glock 19 stock pistols with the 5 lb trigger and a very stockish M4 rifle with the carry handle or a BIUS flip up rear sight only - no optics. Being able to use irons is very important because, as stated earlier, they don't break or fall off, or run out of battery power.
That said, the advantage you get at ANY range with either a close range dot or a magnified combat scope cannot be disputed IMO. Even at very close range, a small dot like a Dr. Optic, or even just a big dot front sight a la XS sights on a pistol makes the acquisition and sighting phase of a shot go MUCH faster. This advantage only increases during low light conditions as well. At medium ranges, a 4x NSN Acog, or any Acog type sight, like an Elcan or whatever gives the shooter a big advantage as well. VonMessa is correct somewhat in that it may take some shooters a bit longer to acquire their target with this type of magnified sight, but at medium to longer ranges that extra time IMO is well worth the trade off of being able to put rounds on the target with much more accuracy.
Initially way back when I started working in my business it was mainly the contractors that could afford it, and better equipped SF units that even HAD optics on their service rifles or carbines: now every soldier you see has an optic on his rifle I think. In fact, many non American troops in Afghanistan have rifles with built in optics as their primary sights now, so as important as being able to use irons may be, I'd have to say many troops now a days don't even have the ability to use them if they wanted to due to their rifle configuration.