One of the samples in the book uses a relationship to the CRM Workflow entity. The sample is the Workflow Scheduler that allows scheduling workflow's on a specified schedule. CRM 4 had a bug where the workflow would only show in the lookup if it hadn't been published.
The workaround is to set the relationship while the workflow is unpublished.
Microsoft just recently released a hotfix that addresses this problem. You can find details about the hotfix at the following location
https://mbs.microsoft.com/knowledgebase/KBDisplay.aspx?scid=kb$en-us$948126&wa=wsignin1.0