I've never really considered myself an information architect, but I've done quite a bit of work that would fall into that category over the years. Jesse James Garrett has an excellent article titled A visual vocabulary for describing information architecture and interaction design. I've just printed it out and will be perusing offline for a few days.

Both diagramming tools I've used on Mac OS X (Graffle and Glyphix) give me the ability to create custom libraries of symbols, that would be perfect for this. However, they are also both limited demo-ware, so I need to cough up about a hundred bucks for the privilage.

On the flip-side, having to use Microsoft Visio instead is almost enough reason to do it anyway.