I suggest you to find out if you can implement this http://koivi.com/ie-png-transparency/ on WordPress. I am using Firefox, so personally I do not have to look unsupported transparent PNG-images. But it would be nice gesture for all IE users.
I did not say that it is a WordPress bug. I only made suggestion. But yes, what would be a better way to force all IE users to use Firefox than to have PNG images with large transparent areas on pink or green backgrounds. That is actually a good idea. Firefox users could use normal looking PNG images where is hidden messages on the transparent colour saying: USE FIREFOX! ;)
And I also heard that this “IE bug” would be fixed in their next IE7 version. So never mind, who cares, and I am using Firefox as well as I said.