I've posted a tutorial on creating 3D objects in Avalon: http://www.kindohm.com/technical/WPF3DTutorial.htm. It's complete with Mike's crash course in 3D graphics theory, code samples, and screenshots.

One of the goals of the tutorial is to help 3D graphics novices (like me) past some of the fundamentals of 3D graphics theory. I had a hard time finding out any of the basics of the topic when I was looking into 3D in Avalon, and I thought it'd be good to help people gain that understanding more quickly than I did.