The Parallel Activity in a workflow must surely start one thread per branch in order to truly be parallel.  Right?  Wrong.  See Nate’s excellent write up here: http://blogs.msdn.com/advancedworkflow/archive/2006/02/23/538160.aspx.

It’s talking directly about Windows Workflow Foundation, but, remember, SharePoint sits on top of WF so it applies to us as well.

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