Very simple and almost unknown trick on popular Android smartphone.
If you want to take a screenshot on your Samsung Galaxy S, just:
1. Press and hold (not tap!) BACK button.
2. Press HOME button (while BACK button is still pressed!)
and you will hear camera shutter sound and a message "Screen captured. Saved as image file".
SGS saves screenshots in folder /sdcard/ScreenCapture.
Your SGS should have Froyo installed to work with this trick.