Microsoft Expression Interactive Designer ("Sparkle") is available for download.
From the Expression team's new blog.
"So, first, what is Expression? Microsoft Expression is a suite consisting of three products each intended for use mainly by the professional designer community. Expression Graphic Designer is a visual arts package equally at home with vector or with bitmap graphics. Expression Interactive Designer gives interaction designers the environment in which to build Windows applications for the Windows Presentation Foundation platform - in other words, to build the new Vista-wave generation of Windows user experiences. Expression Web Designer is the product which offers all the tools you’ll need to produce high-quality, standards-based Web sites."
Tim Sneath has a quick set of steps that show off what you can do with it once you've got it installed.