In PS6, if you are in Quick Mask and mask off an area and then exit, usually it shows the selected masked area. Now all the sudden upon exiting Quick Mask, the masked area does not show as being selected, only the border of the canvas. I tried selecting inverse and I get something like "pixels are no more than 50% selected". I usually hop out of QM and delete area that is not masked and it is not working, possible change in setting with an accidental keyboard shortcut?
I've never actually used the quick mask feature, but the idea of trashing your prefs file and seeing if that fixes it is a good one.
I read a brilliant idea somewhere, and (of course) have never actually used it... but since PS prefs can get corrupted for no apparent reason sometimes, they recommend trashing the prefs you have now, reconfiguring everything exactly to your liking, backing up a duplicate of that new, perfect, pref file, and saving it somewhere. Then, if you ever need to trash your prefs again, you can immediately replace them with a copy of your 'perfect' back up file.
If I ever need to replace my PS prefs, I'll kick myself for not doing that. ;)