Image transparency for a png image (the full transparency, not alpha transparency) works perfectly for me in IE 6.
That file contains transparency... It displays perfectly for me in IE6 and NS7. You're right, if you have a semitransparent area, it will display at full opacity, but if you have a full transparent area, it will display as transparent, which allows it to function with all the ability gif has. And for the file sizes, as Watchdog mentioned, my sites did use gif. When I switched to png, and using some compression techniques, and limiting the color palette, I saw an average of a 20% decrease in image size on every image. For example, that validcss png above is 889bytes. The exact same image as a gif is 1211bytes.
If you don't believe me, look at the file sizes of that one (gif) and the one posted earlier in this post (png).
