It’s definitely a well-known bug – see here, here, and here. Just to throw a few links:
- This blog offers a workaround using a IE specific technique.
- This blog has a lenghty explanation for the workaround.
And then there’s this addressing a probably different bug in an early release of IE8. He got it solved by enlarging the image – maybe worth a try if you’re working with a 1×1 image.
Maybe you can work around this by using different pngs with different opacities, and no CSS opacity?