Yup. I think it's a TouchWiz only feature because it doesn't work for my friends with non-Samsung Droids. Like you said, just hold down the back button then press the home button. You'll hear the camera shutter sound effect and a message "Screen captured. Saved as image file." Then in your Gallery a new folder will be created named ScreenCapture with all your screen shots in it. I've tried it in several apps and it has worked fine. I sometimes have trouble getting a screenshot in the browser but after several attempts one will work.