Old news, but just read it today, that the upcoming IE 7 will finally support alpha channel transparency for PNG images.
After the filter-hell I had to go through to support them on IE 6 for a project (which goes online soon) I more than welcome that addition (yes, it's possible, see this for some info and it's even possible without JS, but I lost that link)
More interesting links about that topics:
What's new in IE 7 (by Microsoft).
CSS Hacks and IE 7