I love targeting mobile devices for clients that receive phone calls. When this is the case I setup up mobile devices to only receive phone calls. No need to worry about conversion rates of sites, every click = phone call. Whilst the cost per click is higher the cost per lead just can't be beaten.
@Jayson - Thanks for that thought.. I have to work out some details such as how to only serve ads when someone is there to answer the phone or perhaps program the landing page to show a phone number during work hours and a form after hours.