Vista provides directory virtualization which redirects file writes to a virtual directory when using UAC (User Account Control). Since we don't have the ability to check filesystem permissions on win32 (possibly elsewhere as well) the EM and Software Update create a directory or file to check if we have write access. This will always succeed on Vista when running as an admin with UAC.

I might be missing the big picture but this description is inaccurate. "This will always succeed on Vista when running as an admin with UAC." <-- Not true for processes with an embedded Vista manifest. Processes without a manifest will be run in "compatibility mode" by Vista and writes to locations like Program Files will be virtualized & will always succeed. If however you mark your process with a manifest as "Vista-aware", you will get your ACCESS_DENIED error when trying to write to a restricted location.

There are also several things that need to be considered such as iirc a bug on XP where the OS would reboot when a manifest was provided, that we only support compiling with VC6 and VC7 on the branch, we aren't providing manifests on the branch, etc.

The XP/Csrss.exe bug is well-known and can be worked around easily, see http://support.microsoft.com/kb/921337 You can add a manifests with any compiler - after all, it's just a resource with a fixed type and ID. You can't expect to be able to provide Vista-compatibility without an embedded manifest anyway - without manifests the appcompat layer will keep second-guessing your application until you're ready to tear your hair out. Once you do add a manifest, things will start to fall into place. Certain problems, like this bug here, will go away immediately.

Sounds good, would you be willing to take this bug and fix it?

I verified that the add-ons mgr. is now behaving correctly on Vista since bug 378598 has been fixed. The uninstall button for DOMi is now disabled. Resolving -> fixed by the patch in bug 378598.