While working on achieving the best possible crossbrowser compatible experience, going as far back as Internet Explorer 6, I was excited to see a site called Tinypng.org. It’s aim is to create a smaller version of your png assets while preserving transparency as it converts the file from 24 bit to 8 bit. It does these things very well and it even works in IE6. Yes you can have working transparent PNGs in IE6, as long as they are of the 8 bit variety. But there’s a gotcha. The transparency it supports is binary alpha, or in other words, each pixel will be fully transparent or fully non-transparent. Yes, that’s just like your basic GIF image.
To get an opaque png working in IE6 you could try the alpha image loader filter, but frankly, I think it gives you more trouble than it really is worth. But hey, it’s 2012 and global IE6 usage is down to around 1% or less in most Western countries. For the majority of websites, IE6 shouldn’t be an issue anymore.