I recently had a question from one of my students:
Every time I create a Visio diagram for a SharePoint farm, I start with an existing diagram and use the shapes from that. Are there any Visio stencils for SharePoint?
Yes! There are! You can download a set of Visio stencils for architectural drawings of SharePoint farms from Microsoft here: http://bit.ly/spstencil.
The stencils let you produce diagrams like this one from TechNet: http://technet.microsoft.com/en-us/library/cc263199.aspx
You need to unzip this to your My Shapes folder under your Documents folder, e.g.: