onOne DSLR Remote
onOne Software offers a iPhone app that allows a tethered camera to be triggered from an iPhone talking to the PC via a WiFi connection. The PC (or Mac) server based application is provided free of charge, The iPhone app is available in two versions, DSLR Remote Lite (free) and DSLR Remote Professional ($19.99).
So how does it all work?
The PC server version of the software is downloaded from the onOne Software website, Once the software is down loaded and installed, it can be started and configured. One item to be configured is the storage location for the images captured by the DSLR Remote software.
The iPhone app is available at the iTunes Store.
The camera (Canon 20D in my case) is tethered to a PC and communicates with the DSLR Remote Server application. The iPhone application then connects with the PC via a WiFi connection. It is therefore necessary for both the PC and the iPhone to be connected to the same WiFi network.
With the DSLR Lite app on the iPhone the camera basic information can be displayed on the iPhone. This include the camera's Focus mode, program mode, shutter speed, aperture, white balance, ISO and the last image captured. The professional version of the app allows the cameras settings to be changed remotely from the iPhone and also supports live view display. The professional version also provides intervalometer and exposure bracketing functions.
So does it work?
Yes, it does. I've tested the DSLR Remote Lite app controlling my Canon 20D. Everything worked, no problems and no flaws. I will be testing the professional version in the new year.

This would work great in a
Since this solution requires
Since this solution requires a laptop and a iPhone/iPad, you can use an adHoc network connection using just the laptop to provide the WiFi connectivity.