Now, that I want to start using Visual Studio.NET 2008, I need to get this working.
To do so, go to your C:\Users\<User>\Documents\Visual Studio 2005\Addins and copy all the files that start with CopySourceAsHtml (there should be four, a pdb, a dll, a config file and an add-in manifest) to the C:\Users\<User>\Documents\Visual Studio 2008\Addins directory.
Then, in the Visual Studio 2008 Addins directory, open the CopySourceAsHtml.AddIn file, and change the HostApplication Version nodes to the 9.0 version. See below.
Restart Visual Studio.NET 2008 and you should be all set.