Didn't we already get tired of this, like 5 years ago?
Got QuickTime? Got IE? Right. Go here: http://www.w3.org/Graphics/PNG/inline-alpha.html. You should see something like:
Ok. Now go here: http://www.w3.org/Graphics/PNG/alphatest.png. It's the URL of the image shown above. Chances are, you'll probably see:
Doesn't quite look the same, does it? We appear to have lost some transparency. Let's try right clicking on it:
Ah. QuickTime.
Come to think of it, I did just upgrade iTunes the other day. In fact, I remember very deliberately clicking to not associate file types. And indeed, if I look closely in the registry, my .png file associations are untouched (and double clicking a file opens into Windows Photo Gallery). If I look in Vista's rather nice new file association window, I see that it doesn't allow me to reassign my .png associations back to IE, or even to QuickTime! According to this UI, QuickTime shouldn't be handling .png files.
And yet, it is. It's handling the image/png MIME type - by installing an Internet Explorer plugin. (It'd be nice if the Vista file association UI could in future be extended to include MIME associations.)
A quick spelunk through the HKCR\MIME\Database\Content Type tree of the database reveals quite a few instances of {4063BE15-3B08-470D-A0D5-B37161CFFD69} - the QuickTime plugin. I certainly didn't ask for these to be associated.
Fortunately, there's a quick fix - fire up the QuickTime control panel applet, get to the Browser tab and hit "MIME settings". You can now choose which MIME types you want the QuickTime plugin to handle (there are a bunch that Windows can't handle out of the box).
But you know what? I'd already done this last time I updated iTunes. And still it stole them back again. And what's more, going into the control panel app, it played innocent and claimed that these MIME types weren't associated with QuickTime! A quick toggle of a setting on and off and an apply later, and all is well with the world again:
So what's worse? That Apple don't honour MIME types as well as they do file associations or that they ignore your settings and install something that doesn't work as well as the native implementation?