The University of Saskatchewan has been using classroom response (clicker) technology for several years. Now one vendor, Turning Technologies, has released an application that lets you use your iPhone or Touch as a clicker instead of buying an actual clicker.
This only works when instructors are using the web-based clicker-hosting software to gather clicker information. At this time, the U of S has not licensed that technology. We are still using the older radio frequency technology that requires actual clicker hardware. So don't go out and buy an iPod Touch for this because you won't be able to use it on campus ... yet. ITS is investigating the new technology.
Also, you still have to pay to register you iPhone as a clicker. While the initial registration is cheaper than buying a clicker, you must renew this registration every year. So in the long run, it is more expensive than purchasing a clicker.
For more information see http://campustechnology.com/articles/2009/01/06/turning-tech-brings-classroom-response-system-to-iphone-ipod-touch.aspx