I have a similar problem but I checked under System Preferences/Keyboard & Mouse/Keyboard Shortcuts and the shortcuts are still activated under Screen Shots, and hasn't been assigned to anything else but it still won't work. I can't figure out why it won't work.
I use command-shift-3 and -4 frequently in 10.5.8.
Look in System Preferences/Keyboard & Mouse/Keyboard Shortcuts and check that they are still activated under Screen Shots, and also check that they have not been assigned to something else.
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Can't take screenshot
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