David Chappell wrote a whitepaper about human workflow (i.e. approving a document) and its support in Windows SharePoint Services 3.0. The Office SharePoint Server 2007 comes with out-of-the-box scenarios for workflow.

The client I just finished with has a need to do some "long running workflows" that involve human interaction. Rather than stuff this idea into WCF services, BizTalk, etc. -- it sounds like SharePoint Server might be a nice alternative.

Blog entry and whitepaper here.