Then there's people like me that just plain don't like IE7. To this day the only copy of IE7 I have is on my flash drive, so that I can test websites. I use IE6 on sites like microsoft.com where they pretty much require you use IE, and everywhere else I use firefox.

Also, PNGs take quite a while for dial-up users to load, even if they're interlaced. As far as transparency goes, I don't see it as being that important.