Ian on Zooming XAML
Ian Griffiths posts the beginnings of a XAML editor (it supports squares and elipses and saving, but not loading XAML, so no source yet). Especially interesting, however, is how he handles zooming. He wrote an explanation with source on that bit.
BTW, if you haven’t figured it out yet, Ian is quite the Avalon boy. If you’re into such things, you should subscribe to his blog.