Press and hold home and tap the power button. This is the magic key for Samsung Galaxy S2 to take a screenshot. It has been such a good news for users like me who used to use iPhone. Somehow it is just the same as the iPhone setting. Actually some of the recent released mobile phone adapt this feature too, including Samsung Galaxy S2, LG Optimus 2X, LG Optimus 3D, HTC Cha Cha. Enjoy!