In our Mac OS X 10.6.3 troubleshooting guide, we noted a number of dual-monitor display issues that occur for some users after the update. In the most common case, external monitors no longer function at the desired (usually highest) resolution. The external display remains blank until the resolution is switched to a lower setting from the Display panel of System Preferences. In other cases, the external display (or primary) will not function at any resolution, or the display flickers or exhibits other distortion.
This problem can usually be resolved by changing resolutions or reverting to Mac OS X 10.6.2.
It now appears that at least some instances of this issue can be resolved by deleting the file /Library/Preferences/com.apple.windowserver.plist then restarting.