Buried deep in 1up's (admittedly fascinating) hands-on with Metal Gear Solid 4 is a little tidbit of info on the upcoming rumbling Sixaxis controller for the PlayStation 3.
Apparently, it's more than just a new Dualshock; the controller's rumble has become more precise: "As explosions rattled around Snake, we could actually feel the direction from which they were coming based on what part of the controller vibrated."
So more than just an on/off switch for the rumble pack. Sweet.