Look at this JSFiddle: http://jsfiddle.net/vkNtK/1/
In IE9 those two h2's render the same, and in all other modern browsers the first one uses the light version of the font. Does anyone have a solution to force IE to use the lighter font when font-weight:lighter; is declared?
To be clear: we're talking about IE9, why isn't this working in IE9?
This appears to be a problem with this specific font, or with the way Google makes IE use it. Your code works for other Google fonts with Book (300) font weight, like Lato and Open Sans.