You could use your thumbnail to use the circular Select button, but then you may scratch the keys, which I've read can happen.

ENGADGET: Hands-on with Motorola's RAZR V3 for Cingular

This may be conceptually sensible, but this button has a circular arrow symbol reminiscent of the "Back" button on a Web browser.

ENGADGET: Switched On: Jane, Stop This Crazy Thing

Once you've lined up the shot you want, a tap on the circular on-screen button will fire the camera.

ENGADGET: Nikon WU-1a wireless mobile adapter for D3200 review: using Android as a remote trigger / wireless display