Aperture 3 supports tethered shooting with a host of Canon and Nikon DSLRs. But iOS devices work quite well too. Simply connect an iPhone or iPad via the USB cable to your Mac, then start a tether session in Aperture (File > Tether > Start Session).
A tether session in Aperture using the iPad mini as the camera.
When you tap the shutter release button on the device to take a picture, Aperture grabs it and displays the image. This setup can be handy for all sorts of situations, such as product shoots and portraits where the large composing screen makes the job easier, or in the classroom with lots of kids. And since the built-in camera is actually quite good with the iPhone 4S and iPhone 5, plus the latest iPads, the main limitation is the software controlling the camera. The built-in Camera app is basic at best...