Win 7 64 Pro
Quad core Intel
No problems with LR3.x or LR4
Started after LR4.1RC
Will not load PS after some (not sure how many) roundtrips to/thru edit-in NIK Color EFX 4 or PS
Must reboot to continue.
Note: When this happens cannot start PS outside of LR until reboot.
This is a very old old issue with PS that has never been fixed (although I can remember multiple reports about it) : under some circumstances, when you "exit" PS, the main window will close but the process (photoshop.exe) will remain active (you can see this in the Task Manager). So you can't re-launch PS because only one instance of this process is allowed. You have to kill the photoshop.exe process in the Task Manager and you'll then be able to re-launch it.
This problem is hard to reproduce but I noticed that it can happen when launching PS and then exiting after a while without doing any work.