Well, that took only half a day of fiddling...
Since the Windows XP Service Pack 2 includes a significant update to the Tablet PC part, one of the first things I installed when my tablet finally arrived was installing the SP2 public preview (RC2 at the time). I noticed a few bugs, and was happy to see most of them gone in the RTM version I am running now.
One of the odd things I noticed was that SP2 broke the transparency of my PNG files. I am using a HTC component to properly display PNG transparancies on a couple of websites. The neat thing about this approach is that it requires only a single reference to a CSS file, and there is no additional scripting that could confuse other borwsers. However, after installing SP2, transparency was gone.
Looking further, I came across the most ambitious HTC project I know of: Dean Edwards' attempt to fix all current CSS problems in IE. While his PNG workaround demos seem to have some quirks, most of his fixes, implemented in HTC's worked just fine. So it wasn't just the CSS behavior mechanism that got hosed in SP2.
When I tried to get Dean's package to work from my webserver, non of his fixes seemed to work. Not from my servers, but on his, they work fine. The exact same files, the exact same browser. Argh!
Back to some of the other Service Pack 2 bugs, or features if you are so inclined. I guess someone decided that in orded to increase security, IE now has to be really strict about mime-types. I notcied on some of the weblogs I visit, just clicking on a link to an asf would often no longer open media player as we have all come to get used to. If the mime-type is not properly configured, you're getting the file as text/plain. Or as it turns out text/jibberish. IE is nice enough to warn me that the file might not display properly.
See the link with the HTC problem? The exact same files, the exact same browser, different mime-type. HTC files are now required to be of mime-type text/x-component, or they just won't work! Dean Edwards already tells us that we really should use the proper mime-types (near the bottom of the page), but many other people haven't done so.