Same for me.
Laptop and additional monitor:
MacBook Pro late 2009 running Lion 10.7.5, updated
LED Cinema display, 24 inches, Late 2008 NVIDIA GeForce 9400M 256 graphics
The LED display started flashing off & on (to black -- it's not the alert flash, which is turned off in sys prefs). Changing monitor resolution did not fix it. Shutting down and re-seting the video cable plug did not fix it. The laptop display is not affected.
I restarted and reset PRAM multiple times. The on-off blinking continued for several minutes while I started to research the issue. Now, as I am writing this, I realize the blinking stopped after about five minutes or so.
What might be going bad?
Posting this under hardware, too.