Not available for VB until next version of VS (Whidbey). There is a project out there that does this though, if you want to look for it. Try source forge, or the gotdotnet project workspaces (I think that is where it is at).
After you build your DLL, you can add the comments with this tool, and save them as an XML file.
It does require that the XML file ships with the DLL, which is kind of a drag. But the up-side about is, that you don't need to redo your comments after a rebuild. All you have to do then, is add the new comments.
I wish I could think of something witty to put in my sig...