First let me throw it out there that I am running CS6 Cloud Suite under Mountain Lion on a Mac Pro. I have no other versions of Photoshop installed, and I've never HAD any other versions installed on this machine.
The problem is this - when I right-click, say, a JPEG file and choose to open with Photoshop CS6, it will launch Photoshop as expected. But if Photoshop is already launched and I right-click to open, say, a TGA file, it will spawn a completely new instance of Photoshop to open it in. From that point on, with the two instances of photoshop running at the same time, right-clicking and opening any other files will open them in one of the two PS instances.
It's almost like some file types have been associated with one "instance" of PS, and other formats are associated with the other. This seems impossible since there's only ONE install of Photoshop on my computer.
That one you need to report to Apple -- Photoshop itself has no control over how the Finder launches it to open files.
Finder decides which application to open, and which copy of the application (or new instances).
Photoshop normally tries to prevent new instances from being spawned - so I'm not even sure how Apple pulled it off...
Obviously not the answer I wanted (Apple support is never going to answer me on this one!) but I really appreciate the quick response.
Apple might eventually get Mountain Lion straight by about version 10.8.6, if they take it that far. They never put out enough dot releases to get Lion right.