Line 5 of omega/omega/css/omega-text.css is setting the 'text-decoration' property on a:visited. It seems that you can only set the color property.

http://hacks.mozilla.org/2010/03/privacy-related-changes-coming-to-css-v...
http://msdn.microsoft.com/en-us/library/ie/hh180764(v=vs.85).aspx (see SEC7115)