Originally Posted by RobertK

Part of the difficulty with IE displaying PNGs correctly has to do with the built in gamma settings, not just a lack of transparency support. IE7 appears to fix this, but anything lower will experience this problem with the gamma settings. One fix for that is to strip the image's gamma part and set it to zero, which solves that trouble. See more here.

So unfortunately you'll probably need both fixes if you want to use PNG images.