I am using Qt 4.6.3 with MinGW on Windows to build Qt apps and now need to add a COM interface to my application. I enabled ActiveQt but was getting post-link errors because I was missing a copy of the MIDL compiler. I downloaded a copy of the latest MS Windows SDK, which includes MIDL, but now MIDL complains it cannot find cl.exe. The only conclusion I can draw is that you can only build ActiveQt applications using the MS compiler, which I would rather avoid. Is a way to get this working with MinGW or am I out of luck?
Well, you can build ActiveQt with MinGW, but using a bunch of COM stuff on top of that may not be possible, because it may or may not be present in MinGW. Some thoughts:
Using any MS SDK tools with MinGW won't work (exception is mingw.org + DXSDK which should work most of the time).
Are you sure you are linking all necessary libraries when compiling? I can't help more if you don't show the exact error messages.
The mingw-w32/w64 project tries to provide a completer "Windows SDK for GCC"; it may contain the libraries/files you are looking for. They provide a x64 and x86 compiler, and pretty good DX support. I have no experience with their COM stuff, but I believe it would be a bit more complete than mingw.org's. You can always contact the developers on the forums or mailing list, they are very helpful.
You could try the Wine implementation of midl, widl. See the Wine wiki page regarding building on Windows.
If you want to give it a quick run, get wine-prgs-0.9.14-mingw.zip and see how it works.