The source code of the 3D engine called Balder is available on CodePlex , now. The state of the source code is not 100% yet, but people can download it for now and have a look at how you could do a simple 3D engine.
A live demo of the 3D engine  is available online. Read more at the Blog'A'Little blog . Well, I hope texture mapping will get reality next. To run the 3D engine you need Silverlight 1.1 alpha .