Right after upgrading to Snow Leopard, John Carter discovered that he could no longer use the keyboard shortcut (SHIFT-CTRL-CMD-4) to take a snapshot of an area of the screen and paste it (CMD-V) into a Mail message (the snapshot is saved in the clipboard). However, he could paste the image into any other application. To verify the problem, John logged in as another user on the Mac and performed the same operation. The problem did not show up in the other user's login environment. So this meant that the problem was isolated to John's login.
Taking some lessons from an earlier call to Apple support, John began troubleshooting the problem by first working in the ~/Library/Preferences folder (the '~' character is shorthand for the home folder, or in this case, /Users/jcarter). After a short session of trials which involved pulling files out of ~/Library/Preferences and testing with Mail, John discovered that when the file "com.apple.screencapture.plist" was removed, the problem went away.
When a file is removed from the ~/Library/Preferences folder, the next time the user logs in, that file is recreated when the function/application it was designed for was again used. Of course, any preferences (other than the defaults) for that function/application are lost and have to be recreated. Removing the preference files is the basis for doing the test to determine which preference was causing the problem; remove a preference, log out and log back in, and perform the test again. If the problem goes away, the preference file removed was the culprit. If the problem persists, replace the file that was removed and repeat these steps until the problem is isolated.
John discovered to his amazement that the com.apple.screencapture.plist file was not recreated after performing the screen capture (snapshot) operation. This strongly suggested that that file is not used or needed in Snow Leopard.
So John called Apple support to confirm this finding. Sure enough, Apple support says that file does not exist in Snow Leopard.
That file may have been created in John's login environment by a third party application on John's machine (but definitely not by Jing - confirmed). Apple support cannot find any record of that file existing on any of their Snow Leopard machines.
It is recommended that if you have the file ~/Library/Preferences/com.apple.screencapture.plist, you should remove it.
John's closing advice, "Don't be concerned if any of this is confusing. If your Mac is working the way you want it to, pay tribute to the Apple gods."