Microsoft have released an early draft of the XAML and WPF specifications. The XAML specifications are divided into two parts: First is the Xaml Object Mapping Specification. This defines the XAML language itself and focuses heavily on parsing. The second specification is the WPF Xaml Vocabulary Specification, which describes the WPF file format.
The specs are released under the Microsoft Open Specification Promise license, which basically says that if Microsoft has a patent on a covered specification, then the patent will never be enforced against any implementation of said specification.