Wpf and xaml has been out for a while now, so the community already knows the do’s and don’ts of programming in xaml.

Thankfully, there are always some nice people who share this stuff with us.

This is a good resource and starting point. It also includes links to the channel 9 videos that give you some background info on the rationale of these guidelines.

Some other guidelines/info:

Designing with WPF

XAML Guidelines for Creating a Composite UI