Today’s batch of Windows Updates included a “Recommended” update to improve the rendering of certain fonts at small sizes (8-10pt). The updated versions of Arial, Verdana, and Tahoma fonts include new hinting logic that renders more clearly using sub-pixel-positioned ClearType text rendered using the DirectWrite APIs used by Internet Explorer 9's hardware-accelerated text rendering.
I put together a simple test page for these changes here: http://www.debugtheweb.com/test/ClearType/. Using the comparison widget, you can see that Arial's c gets a little wider, dots on i and j get a little bolder, 10pt Verdana's 5 changes a bit, 10pt Arial's 0 gets a bit skinnier, and many of 9pt Verdana's characters gain a bit of weight.
PS: The Firefox guys wrote a bit more about this as well.