Fortunately (or unfortunately?) there's only one browser in use today that doesn't support PNG alpha and it's IE6. Anyone doing anything on their page to try and support anything older than that (IE5.x or Netscape 4.x) is only doing themselves and the rest of the world a disservice.

All versions of IE require a conditional to tune the styling. The PNG javascript fixes mentioned earlier should all ignore newer versions of IE, though you can also make a conditional specific for IE6 (which I do just so I can toss any other daft styles in there).

I'm going ahead with my idea of calling out IE users and have just started to make a style for a block element that will only display on IE. I'll likely make some way to hide it by clicking some part of it, but for now it will be static at the top of all pages using a new template I'm working on. It's going to spell out in no uncertain terms that IE sucks. And provide links to proper browsers.