Hello,
Galaxy Ace has a different method now. Although you have to try several times. Press home button + on/off button, wait just a little, then release both of them at the same time. Notice: most of the time Task Manager will pop up, that's not what you want. You have to be a little lucky to be successful, at least patient until you got a screenshot. And don't press the home button until it vibrates, because then you'll be sure to get the Task Manager. I manage to get the screenshot every now and then.