A Japanese gamer that got the 3DS Slide Pad early cracked open the manual to find that it promises 480 hours of play time from one AAA battery. That's a crazy number, but I'm guessing that this single control doesn't need much juice. Hell, I'm surprised that it can't just draw power from the 3DS.
It's a good thing that the battery lasts that long. A photo on Andriasang shows that the battery compartment for the Slide Pad is sealed with a screw. Who wants to pack a screwdriver? Great idea there, Nintendo.
The Slide Pad also comes with a strap. Neato.
It's too bad that they didn't make the Slide Pad an additional battery to add to the 3DS's battery. I find myself not wanting the Slide Pad as it would force me to take off my beloved Nyko Power Grip for 3DS.
More play time > second slide pad.