I've checked for the Known Issues, verified that the file is accessible, served with the right content-type, etc.

It does appear to be doing SOMETHING - a single drop-shadow is visible with it and not visible without. However, none of the gradients and backgrounds show up here:
http://br-tool.de/demo/info

So I'm now out of ideas why it's not working.

But you did not check these issues/documentation:
http://css3pie.com/documentation/known- ... #shorthand
http://css3pie.com/documentation/suppor ... background
So: shorthand only and use -pie-background for your gradients.

And hey, be careful with your use of ems with your elements using PIE: http://css3pie.com/documentation/known-issues/#elements

I checked those. Apparently I misunderstood.

I use the shorthand syntax, but I also have the browser-specific ones. It really is shorthand only, i.e. if I have browser-specific ones, it fails?

And from the demo, I was under the impression, that the -pie prefix is a fallback, in case the CSS specs change.

I'm aware of potential problems with using ems, but my love for IE only goes so far and I'd rather accept display problems on IE than sacrificing resolution independence for everyone else.

Browser specific properties are totally ok. Only the standard border-radius needs to be shorthand.
Let's agree to love user experience and paychecks - not IE "Let them eat square corners"

Ok, understood.

I do have non-shorthand in one place (where I only want 2 corners rounded), but everywhere else it uses shorthand.

I'm currently using a different approach than pie, but it would be cool to have pie working as well. *shrug*

But you can do only 2 corners with border-radius: 0 4px 0 4px for example. Are you having any other showstoppers besides the use of ems?