In the past, screen captures on Android have meant rooting the device. Some devices had special key combinations that would allow for a screen capture. Google just didn’t include this as part of the operating system, so some devices would do it and others not. For the devices that you could do screen captures on, the key combination varies from device to device.
Enter Ice Cream Sandwich. The latest revision of Android brings screen capture into the base OS. Hopefully this will bring standardization across the platform and going forward all devices will have the ability to do screen captures with a standard key combination.
On the Galaxy Nexus, you can do a screen capture by pressing the volume down button and the power button at the same time. Within a second or two, you will see the screen capture and then a notification in the top bar that it is being saved. Jump into the gallery and you can browse through all your screen captures on your device. From there it’s just a hop, skip, and jump to posting it on damnyouautocorrect, or sharing with your friends on Facebook and Twitter.