W00t - they are now at 4.7 - 5.1% of the total PC market in the U.S. depending on who you ask:

http://www.informationweek.com/news/showArticle.jhtml?articleID=196901953&subSection=Breaking+News

Those are still not numbers that make it an attractive enough platform for malware writers to seriously target if they want good ROI in my opinion . . . but if Apple can sustain that growth - maybe in a few more years it will be. :)