A curious gamer by the name of PuyoDead over at GameFAQs decided to crack open a Nintendo DS Rumble Pak to see what makes it tick. The conclusion based on a cursory examination is that it uses a different rumble method than other rumbling controllers. That makes sense, after all. It wouldn't do any good to send the DS shaking so hard that it impedes gameplay. For the record, I picked up Metroid Prime Pinball this week and the Rumble Pak is not loud or intrusive. It gives off a slight jolt when various events occur, such as a bomb exploding or a gun shooting, but it is not distracting or annoying at all.
I'm looking forward to seeing just what creative developers can do with the add-on, but I have to say that one aspect of the Nintendo 64 Rumble Pak is missing from the experience. It's a silly, stupid thing, but I always liked how when I was about to play Goldeneye 007 I'd load the Rumble Pak into the N64 controller as one would snap a machine gun clip into place. It provided a touch of "fourth wall" realism that one does not get by sliding a game pak into place.