I recently upgraded my Samsung Galaxy SII to Ice Cream Sandwich (yes, I was procrastinating). After doing so, I found Samsung's built-in screenshot-taking feature to be (very frighteningly) gone. While before I would press the power and home buttons at once to take a picture of the screen, now that was no longer working.
Thankfully, Google built taking screenshots into Android 4.0 (and later). To use this feature, you have to hold the power button and the volume down button at the same time--which requires both some good time and patience. I found that if you long press the buttons simultaneously there's less of a chance of your phone just turning off your screen or lowering the volume.
If you don't have ICS, there are still lots of other ways to take a screenshot on Android.