I noticed this a few years ago when we started to use the ACOGs intensively. I didn't know the name (I just referred to it as "crossed-eyedness"), but I definately knew the result.

I have never recommended the use of the ACOG as an OEG, mostly because of this distinct shift we see in about 50% of shooters to varying degrees and distances of effect. I use the OEG technique only as a method to get the shooters into shooting with both eyes open.