Hmm, I would have thought it would need to be quite a strong magnet to keep something the weight of a mouse attached - wouldn't it make it quite hard to actually move the mouse?

From their FAQ (basically magnet only holds mouse in place when not in the middle of mousepad):

WON'T THE KEYBOARD AND MOUSE FALL OFF THE DESK?

The desk has a layer of steel under its surface. Magnets on the keyboard and keypad hold them in position on the desk. We provide specialized magnet kits for standard Apple products and a general-purpose kit to accommodate other products. The mouse pad has a ring of steel on its outer edge but no steel in the center. This allows the user to slide the mouse easily when mousing and store the mouse anywhere on the edge of the pad when not in use.

Thanks, I don't know how I missed the FAQ - I was struggling to imagine a solution that wouldn't be annoying, but what they've come up with actually sounds pretty good.