Internet Explorer 6 does not properly display alpha transparency in 24-bit PNG images. Many sites, however, use a CSS/Javascript "hack" as a workaround. The problem with that fix is that some IE users, for currently unknown reasons (possibly some kind of security setting), are only able to see any 24-bit PNG images while they are loading; once they've loaded, they seem to disappear. What really happens is the image is replaced with single-pixel transparent .gif image usually named "blank.gif", although I believe this site uses one called "spacer.gif".
A great example of a site where this is incredibly obvious is
DeviantArt, as the issue renders many, many gallery items unviewable.
Fortunately, this site is different from others that use the same "hack" in that it apparently checks for IE 5.5-6 specifically, as I can see the medals using
Internet Explorer 7 (Beta 3)--which has full PNG support, but can still suffer from the same "disappearing-image" issue. Elsewhere, you'll just have to wait until IE7 is officially released, and/or the applicable sites wisen up about it.