This is How to take screenshots on Samsung Galaxy S2/S II I9100

 Here's a quick tip on how to capture or take a screenshot from your Samsung Galaxy S II. Most android phones could only capture what was on-screen using  tools, or applications and many required root access first. The Samsung Galaxy S II has a Built-in screen capture, so taking screenshot is too easy and doesn't need any rooting or 3rd-party application to install.

Here is how simply you can do it: PRESS AND HOLD THE HOME BUTTON - THEN PRESS THE POWER BUTTON. So, press and hold home, then a millisecond later hit the power button. Works flawlessly every time. The image will then saved in your gallery files.

Update: 

The ICS already released, taking screenshots for Galaxy S2 runs with Ice Cream sandwich is change to  Volume Down and Power Buttons..Press and  Hold The VOLUME Down and POWER BUTTON simultaneously  to take a screenshots.