Why not have a go yourself?

If you have a Mac laptop, you've got yourself a mobile usability station. Just get a copy of Silverback App ( http://silverbackapp.com/ ) and start usability testing!

Following on from that, if you'd like to make the most of your usability tests I'd recommend getting "Rocket Surgery Made Easy" by Steve Krug which will run you through the general process: http://www.sensible.com/rocketsurgery/index.html