The feet are the button →

Interesting tidbit from Macworld’s Magic Trackpad hands-on:

Despite the Magic Trackpad’s thinness and its support for the same tap-to-click feature as MacBook trackpads, you can also click by pressing down. The trackpad surface itself doesn’t depress, though—instead, when you press on the trackpad, the force from the surface it’s sitting on will depress the two round feet at the trackpad’s base. That’s what registers a click.