Here's how to get the Word Merge control working on Windows Vista/IIS7.

1. Install the Office 2007 (if you have Office 2007 installed) or Office 2003 (if you have Office 2003 installed) Primary Interop Assemblies. These can be found in TFS in the InteropAssemblies folder of any given branch, or downloaded from Microsoft if you don't have TFS access.
2. Give the Trusted Sites zone FullTrust in the .Net Security Configuration tool.
3. Add the iMIS website to your Trusted Sites list in Internet Explorer.
4. Follow the steps given in this MSKB article, except in steps 5-9 you want to edit the iMIS web.config file, not any config file under %windir%\system32\com
5. Add a virtual directory from http://server/virtualdir/downloadbin to the \Packages\Platform\iMIS\DownloadBin directory. (This step can be skipped if you installed iMIS from an installer instead of by hand)
6. The Word Merge control should now launch correctly. If you discover additional required steps, please add them.