Thanks to Warner over at GottaBeMobile.com for the tip on this. Graphite is a drawing program that allows you to generate Visio-style diagrams using digital ink. I use Visio a lot in my work and, frankly, it's not a very Tablet PC-friendly application. Graphite looks like a great way to augment Visio with and ink-friendly environment for capturing ideas quickly.
More information can be found on the Graphite blog.