I understand that earlier versions of Android do not support a native screen capture feature, unless provided by the specific device, or you have a rooted device, or you have the Android SDK and link via USB. However, I have read that ICS should support a native screen capture, but the suggested volume down + power combo in that article does not work for me.
I have also tried home + power, as seems to work on other devices, but that does not work for me either.
The official Android Developers site for 4.0.x states that this should be possible, but doesn't say specifically how...
Sharing with screenshots
Users can now share what's on their screens more easily by taking screenshots. Hardware buttons let them snap a screenshot and store it locally. Afterward, they can view, edit, and share the screen shot in Gallery or a similar app.
I have a generic (DGM T-703) 7 inch tablet with ICS (Version 4.0.3)