I don't know why Android manufacturers make it so difficult with taking screenshots, either blocking you from taking them all together or using a different method all the time.
Thankfully ICS and Jellybean now have a standard way of taking screenshots which I hope will stay the standard throughout all Android devices, as it the two versions of Android are now making their way onto more and more devices.
How To Take A Screenshot on Nexus 7
This is pretty straight forward but not very obvious. You have to hold down the volume down button and the power button at the same time for short period until you see the screen make an animation, showing a screenshot of your screen.