Can you use the screen on a Samsung Galaxy S as camera flash for the front facing camera?
Even on models that have a built-in flash, using it isn't worth your time.  Point flashes almost always make the picture look bad, especially when the camera is a crappy phone camera.  I suggest making sure the subject is well-lit instead.  That all said, the screen isn't bright enough for a flash.  Best case it would provide a little "ambient" lighting in a dark room and you'd have to take the picture up close.