The galaxy tab will use a capacitive screen, iirc. I would guess that its implemented in such a way that it uses bluetooth to tell the tablet when it's in contact with the screen (it's detecting the capacitive field) and so the tablet can reject all other inputs (like palms).

The benefit of a stylus is twofold. First it's gloves on scenarios. Second, it allows the selection of smaller elements without the need to first zoom in on a specific area.