According to the University of Virginia's Information Technology and Communication (ITC), which services the IT needs for most of the campus, 43 percent of first-year students at its residence halls during 2009 were using a Mac.
The figure represents a continuation of a five-year trend that's seeing increased Mac penetration on the campus amongst first year students. Prior to 2004, Mac usage amongst freshmen hovered between three and four percent (with the exception of 1997). 2004, however, served as a watershed year: share increased by four percentage points. A host of theories can help explain this jump. I'd probably point to the release of iTunes on Windows in 2003. While the iPod and the "halo effect" surrounding it had existed for three years, up until 2003 only Mac users were able to fully experience the benefits of hardware and software integration. Or, perhaps it was partly due to the fact that OS X-only Macs began making their way into the market during 2003.
Other significant events that occurred throughout the years include the release of the iPod nano (2005), Intel-based Macs (2006), and the iPhone (2007). But perhaps as influential as anything else during this time frame is the "Get a Mac" campaign. The "I'm a Mac" and "I'm a PC" ads highlighted the benefits of a Mac and contrasted them with the downsides of owning a PC -- i.e., security issues, performance and lifestyle apps.
Data for the University of Virginia ITC is collected by the group's student employees, known as Computing Advisors (CAs), a group of first-year students hired to advise and assist their peers with computing. The data is based on a census of first-year residence halls each fall conducted by the CAs, and can be found here.
Hat tip to Glenn Fleishman.