I've seen a few diagrams on the Sitecore developer sites and but does anyone know if they are distributing the stencils somewhere? I'd like to capture the infrastructure topology at our client.
I haven't heard anything about Vision Stencils for Sitecore. But on an architecture visualization perspective I would recommend you to try Sitecore UML Tool. Its an architecture visualization tool helps visualize existing Sitecore architectures in UML and to create, visualize and deploy new Sitecore template architectures directly from their UML diagrams. It even has the power to generate MSDN-style documentation for any Sitecore architecture.