Now it can be told...I am a developer on Microsoft Expression Interactive Designer aka "Sparkle". We finally announced the product at PDC last week and I gave a presentation on how we built the product. I promised the attendees I would blog the content in a fuller way, as there was simply too much material to cram into a 45 minute lunch session...and I will, as soon as I get caught up from a week out of the offiice.
In the meantime, you can learn more about "Sparkle" from this video: http://channel9.msdn.com/showpost.aspx?postid=115387
Also, I recommend my colleague Manuel Clement's blog: http://spaces.msn.com/members/manodesign/PersonalSpace.aspx