This post was grabbed from the blog of Todd Klindt that is worth looking at.
Here is the excerpt:
Here’s another question I see a lot, “Is there a way to have one document in multiple libraries, but have there be a master?” The answer is “sort of.” In SharePoint 2007 there is a new concept of “Content types.” A content type is kind of like a template for document types you store in libraries. In this case we’re going to leverage the “Document Link” content type. You can use it to create a link to a master document in another library or to any URL in general. In this post I’ll walk you through how to do it.
and here is the actual link: http://www.toddklindt.com/blog/Lists/Posts/Post.aspx?ID=49