Are you using a PNG direct from Fireworks, with all the layers intact? That's usually a bad idea, as most of the information is redundant when opened in a web browser. I've also noticed that image editing software also opens PNGs slower too, so I guess the format is quite complex too.
Despite all the advantages of PNGs, support for it in browsers, especially IE, isn't great. Most browsers will display something, but it mightn't be what you expect. Just did a quick test: a transparent PNG displayed fine on IE 5 running under OSX on a Mac, but displayed opaque on IE 6 under Windows XP. So, for the moment, I can only recommend using PNGs with caution. Hopefully that situation will change.