3.6 IE Transparent PNG Bug
This is more a lack of support than a bug. Although the humble GIF format offers transparency, that transparency is either on or off for any given pixel (either fully transparent or a solid colour). With the PNG format, as well as colour, each pixel carries a separate information channel detailing how transparent it is. This alpha channel enables a pixel to be fully transparent, fully opaque, or anyhere between, offering deigners vastly more artistic freedom. IE6 and below do not support the png image format or alpha transparency, so while these image will render fine in other browsers, in IE6 and below the background will be visible.
Fixes
There was no native support for PNG alpha channel transparency until IE7. However, since IE5.5 there was some support in the form of a proprietary filter called the AlphaImageLoader. IE filters can be applied directly in the CSS for both inline and background images (or by setting the same CSS property using JavaScript). An example is shown below:
img { filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(...); }
Unfortunately this only works in simple cases where images aren't repeated or positioned, and the AlphaImageLoader can be very slow to load. In addition there's a bug that sometimes prevents links and forms working when a PNG background image is used. And because the filter property is invalid CSS, the safest approach if the loader has to be used is to apply the filter selectively using JavaScript for IE5.5 & 6. Personally I would not worry about IE6 not displaying PNG's correctly as it can only be a matter of time before it vanishes.