Plugins install pop-up ad software:
[link|http://www.zdnet.com/zdnn/stories/news/0,4586,5095300,00.html?chkpt=zdnnp1tp02|[link|http://www.zdnet.com/zdnn/stories/news/0,4586,5095300,00.html?chkpt=zdnnp1tp02|http://www.zdnet.co...t=zdnnp1tp02]]
At least until they start doing this for Linux plugins, I suppose.