We're using Adobe's Source Sans Pro font via Google Web Fonts. The fonts show up fine in every browser we've looked at: Chrome, Firefox, IE7 and IE8, but randomly certain sections show up being rendered thinner in IE9.
The font-weight and size for the sections is the same as the rest of the page: 400 weight, 13px font-size.
The other issues I've looked at discuss full-page solutions, but bizarrely enough, this is only happening in certain sections of the page.