The product I've been working on the past couple of years is available in CTP form. And, yes, this explains why my blog has been pretty quiet since shortly after PDC.
Here's my take on the significance of the product. The Win32 UI platform at least 10 years old (arguably 20). WPF is a new start, it is not a wrapper like MFC or WinForms, it is fundamentally new. It takes advantage of and exposes the strengths of the .NET platform and of hardware accelerated graphics, and tightly integrates all the new hotness: vector graphics, animation, 3D, data-binding. This new platform requires new tools, and Expression is such a tool. Expression is also significant because we choose to build the product on the platform (WPF) that we were targeting. This created a virtuous cycle where our experiences improved the platform and taught us what UI designers were going to need when they build their own applications. And it is also a statement about the platform: you could not build Expression-like apps with HTML (Ajax or not) or Flash. It is a deep, rich and productive platform capable of scaling to the largest development projects. Both product and platform are in their sub-infancy right now...it's going to be a fascinating next 10 years.