For years the keyboard shortcuts in the GNOME Desktop Manager have been Print Screen to take a shot of the whole screen and Alt + Print Screen to take a shot of the active window. (See the documentation here.) Something happened in Ubuntu 10.10 Maverick Meerkat that breaks the functionality to take screenshots of the active window. This is documented in Bug #642792.
There are a few different proposed workarounds in the bug report. The one I prefer is to re-map the keyboard shortcut to something else that actually works. This is easily done by going to System->Preferences->Keyboard Shortcuts. Scroll down to the Desktop section and select Take a screenshot of a window.
Make sure is says New shortcut... on the right side. If it doesn't, click on the shortcut again so it does. Now simply press your preferred new shortcut key combination. My suggestions are Ctrl+Print Screen or Meta+Print Screen (Meta is the Windows key or Command key for Mac).
That's all there is to it. You can now Close the Keyboard Shortcuts window.