Internet Explorer supports some extra style features for providing transitions and filters to images. Two of these can help us with transparency in IE6. One is the AlphaImageLoader, which applies an image to the background of any element whose height and width are both explicitly set (or if the position: absolute; style is set) and supports full Alpha transparency. It is supported from Internet Explorer 5.5. The second is Alpha, which can be used to provide Alpha transparency to any element, making the element transparent….
Direct link: Making Internet Explorer use PNG Alpha transparency