I didn’t even know this was a problem, but if it is…and I make no claims that it is…then here’s a fix for it. PNG4EVR!
PNG in Internet Explorer: The Solution
This site is all about a JavaScript-based PNG fix for Internet Explorer 5.5 and upwards on Windows. The fix allows IE to properly render PNG alpha transparency.
I ran into the same problem about a year ago. Posted about it and mentioned a couple solutions I found – which both replace the PNGs with GIFs for IE. The one you found looks like a better solution though and I’ll have to try it out.