Hi Otis and thanks for your swift response (as always).
The "digital signing" for Vista Certification requires files (for example .cab, .msi, .exe, .dll, .ocx) to be signed with a digital certificate file (we have a personal information exchange file with a .pvk suffix) that you have to purchase from a certificate provider such as Verisign. Packages like InstallShield include the ability to digitally sign your files or you can do this manually using MS's SignTool.exe
Once this is done and you look at the Properties of your file you'll see an additional "Digital Signatures" tab that includes your company name, an email address and timestamp.
When I first looked at this I also thought that they meant signing assemblies with a strong name key file.
If you use third party files, like the LLBL Gen Pro DLLs, that aren't digitally signed then you have the option to fill out a "waiver" document (a disclaimer) that explains that the files aren't authored by yourself.