Using Windows Workflow Foundation for UI flow

Matt Winkle introduced a sample solution called PageFlow which shows how to implement UI flow using the Windows Workflow Foundation.  More details about the sample are at Matt's blog.  Shelly Guo has a nice demo of PageFlow on Channel 9 here.  I just watched it.  PageFlow adds a new workflow type specifically for UI flow.  In Shelly's demo, she shows using the same PageFlow for ASP.NET & WPF.  Since introducing the sample, some recurring questions are being asked both internally and externally.  Questions like "Why not a state machine?" and "What about WCSF / Acropolis / Codename 'foo'?"  Matt answers these questions on his blog.