I ran into an issue last week with PNG and alpha transparency. It seems that most everything you read says that IE 6 supports PNG transparency with an easy hack. Used it, worked great ... so I thought.
What the majority of the articles don't spell out is that IE 6 only supports alpha transparency in PNG-32 format. Which unfortunately creates much larger file sizes as compared to PNG-8. :(
IE 7 however will display alpha transparency correctly in PNG-8 like the other popular browsers.
The sad fact is that around 30% of people using the internet are still on IE 6 as of this post. C'mon people upgrade your Internet Explorer browsers already. That or switch to Firefox, it's much faster anyway.