If you're about to diagram your upcoming SharePoint 2013 farm, or want to update the one you've already made, we just released some new Visio stencils for SharePoint 2013. Big thanks to Kirk Evans for making me aware of this.

http://blogs.technet.com/b/tothesharepoint/archive/2013/02/13/get-the-new-office-visio-stencil-with-the-cool-icons.aspx