nodecare

background-image: url('/path/to/image.png') !important;
background-image: none;
filter: none !important;
filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(src='/path/to/image.png');

Pros: That's it - all done. Works in IE6 too.

Cons: AlphaImageLoader can slow down your pages a lot.

Alternative medicine for your IE6/PNG headache

Use Microsoft's implementation of VML instead of Microsoft's AlphaImageLoader filter.