Problem:

I am not asked to write TFS Event Handler web services very often and I can never remember where WorkItemChangedEvent is defined.  I spend more time searching for that missing reference than I do writing the code.

Solution:

For TFS 2010 WorkItemChangedEvent is in C:\Program Files\Microsoft Team Foundation Server 2010\Tools\Plugins\Microsoft.TeamFoundation.WorkItemTracking.Server.Dataaccesslayer.dll.