This article in Fortune magazine highlights WiFi installations at Novell and talks about the value of WiFi at work.  There are claims of productivity increases in the article, although its not a study.  Wireless networking (whether by WiFi or a wide area solution) certainly makes sense for people who spend a large portion of their day away from their desk.  Its a lifesaver for me somedays since I can use time away from my desk, in between meetings, to check email, get messages to my assistant, etc.   Before, I'd get home at 7pm, eat dinner and spend several hours going through the days email.  Now, my email is usually done during the day.