I moved this to a separate post, because PNG deserves it's own section. I was working on cross-browser PNG support, which I got working, thanks to a wonderful article over at A List Apart. I works just beautifully, and I will post the results when I finish up the site. Discuss it if you want, few people work with PNG's, preferring to work with GIF's for transparency, or JPEG's because they're used to it.
Next, you should be aware of behaviours which allow you to quickly add a piece of code for IE. I have a lot of links for PNG but this by far is the most useful: PNG Behaviour