I have been working with David Chappell recently and he has written a nice whitepaper that describes how workflow can be used in Windows SharePoint Services and the 2007 Microsoft Office System. It does a good job of describing the various technology components that can be used for workflow development in SharePoint and Office.
The content covers the value of human workflow, a primer on workflow and SharePoint, a description of workflow in Windows SharePoint Services and then for Office SharePoint Server, and finally a comparison describing how to choose between Windows SharePoint Service and Office SharePoint Server.
If you are building on Windows Workflow Foundation but don't yet have a host application for your workflows SharePoint may be the answer for you.
11 Oct 2006 - Updated whitepaper link to new version with some corrected product names.
18 Oct 2006 - Updated whitepaper link again for download center change.