This happenes often, but not consistently.
I'm using CS6 (regular) with OSX Lion, 16GB RAM.
Once I have a few files open, Photoshop starts "stealing" the cursor focus, which is to say if I go to any other application, as soon as I make one click, PS jumps into the foreground and becomes the active application. The click I made in the other app was applied, and I can see it when I go back to that app, but as soon as I click again, Photoshop pops back into focus.
The only workaround I've found is minmizing PS while I use other apps. Or closing it and restarting it. Sometimes just reducing the program window works, and sometimes not.
Do you have the application frame visible? The focus works slightly different (code wise) when the application frame is on as opposed to off. (BTW, that is not to say what you are experiencing is correct - but rather running with, or without the application frame may solve the focus problem).
I will have a QE contact you to see if we can get steps to reproduce the problem.
Yes, the frame is visible. I work with it maximised all the time and the image files snapped into it, so I can't say if it would happen if I worked with the frame hidden. Minimizing the frame does let me use other apps normally, but since the images are snapped to it, they minimize as well.
I'll try to make it happen without the frame visible.
This is happening to me as well, although I am using Windows 7. I like to alt+tab to jump around applications. If PS is in fullscreen mode (press f 3 times) then it does not allow other applications to take focus over it. It simply switches back to the front app on the next screen refresh (instantly).