That silly fade-in script for the tabs? Uses the opacity filter for IE8 and lower, which turns OFF anti-aliasing of webfonts in same. I encountered the same issue trying to use webfonts with the filter equivalent of linear-gradient not that long ago.
Some freindly advice? Swing a massive axe at all the fat bloated scripting for nothing that's destroying anything resembling accessibility on that site -- particularly the tabs bit since it prevents people from directly linking to content, and doesn't show the full content when scripts are disabled. You might want to review and/or cut down the markup a bit since I'd ballpark that being 8k or so larger than need be... in particular all those DIV wrapping perfectly good block-level containers can't be a good thing, especially on such a simple layout.