I just figured out how to take screenshot in Samsung Galaxy series mobiles running on Android , But I have only tested in my Samsung Galaxy Ace running on Android 2.2.1 .
1.Go to the screen in mobile , which you want to take screenshot of .
2.Press the back button on the bottom right and hold it.
3.Now press the bottom centre button and it should make a sound of camera shutter.
4.Go to Gallery app and you will find a new folder called Screen Capture where you can find the screenshot in png file format.