Today I noticed that the external display attached to my new 17" MacBook Pro Core i7 is not recognized any more after I put the system to sleep by closing the lid. The 'Detect Displays' command from the Displays menu or the Displays preference pane didn't work. Rebooting the system is a possible solution, but not the preferred one.
I tried out some things and then searched some forums, but found no working solutions. Then I remembered that little tool GfxCardStatus from Cody Krieger and fired it up. I switched the active graphics card by selecting 'NVIDIA only' and then hit 'detect displays' and immediately the MacBook Pro detected the external display.
I did a little more testing and could reproduce the behavior; I switched to the Intel card by selecting 'dynamic switching' in GfxCardStatus, put the Mac to sleep, woke it up again and once more the display was not recognized. Now I switched back to the NVIDIA graphic by starting iPhoto (you can see that NVIDIA is active with GfxCardStatus), and the display was detected again. Aperture or any other software, that causes a switch to the NVIDIA card, should also work, I tried it with Aperture (which I mostly use in conjunction with the external display, which also explains why I not earlier noticed this issue).
[crarko adds: I haven't tested this one.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100603094215734