Here's a quick tip to get png's with alpha transparency to show up correctly in Internet Explorer.  Instead of hacking you website, hack the picture! 

Using a utility like TweakPNG, you edit the meta information of the PNG format to display a white background (or whatever color you want).

Open the .png in TweakPNG and insert a chunk named bKGD using the Insert menu.  Then simply double click your newly created chunk in the main window to bring up a color chooser.  Select the background color and away you go.