I'm embarrassed to say I was messing around with external Cinema Display preferences on my MacPro (08) OSX 10.5.6 and AE CS3 and somehow everything is black and I can't get rid of it. I chose Main or something in the video preferences in AE and it seems like a loop... I tried removing the recent prefs in User Lib to no avail. Every time I open AE, the dock disappears and all is black - can't find the menu bar, in fact I can't even force quit AE as the finder is black too. Any help greatly appreciated!
It should be set to "Desktop only" for your OS' UI to be visible. All other settings will blanket the screen in one way or the other, if indeed they are anchored as secondary external displays in your system.
First, I found another folder of AE prefs; removed it and now all is visible again... whew!
Second, my 'video preview' panel shows I can choose "Digital Display" as an output with possibilities for "Main" / (blank) / "Full Screen" / "Raw" - I think it was the full screen that got me in trouble! The (blank) works by outputting to my external monitor via video card, but doesn't really fit the screen too well, so I doubt I'll use it.