Our team was planning on releasing a CTP of System.Xaml.dll in November, but we had to reprioritize the CTP as we had several hot issues that we needed to address.
We’re still dealing with hot issues, and don’t want to predict the date that we’ll get around to it, as we don’t want to lead you astray twice. Minimally, we’ll be in the first beta of .NET 4.
We’re hard at work making this new XAML system a reality and integrated with WPF/WF/WCF…sorry for the delay.