The WorkflowServiceHost loads the workflow definition at the moment it starts listening for incoming requests. This workflow defintion is cached so if you are using XOML based workflows and change the XOML you will need to restart the hosting service to activate the new XOML. The reason for this is that the WCF interface defintion can be part of the workflow itself so changing the workflow could actually mean changing the public interface the WorkflowServiceHost is exposing.

http://msdn2.microsoft.com/en-us/library/system.servicemodel.workflowservicehost.aspx