When you build a BizTalk 2006 project, you might get build output containing warnings such as:
Updating references...
The dependency 'Microsoft.BizTalk.CachingService' could not be found.
The dependency 'Microsoft.BizTalk.DBAccessor' could not be found.
The dependency 'Microsoft.BizTalk.Bam.EventObservation' could not be found.
The dependency 'Microsoft.BizTalk.Streaming' could not be found.
The dependency 'Microsoft.BizTalk.XPathReader' could not be found.
Performing main compilation...
This is because one of the BizTalk assemblies has been added to your project with Copy Local set to true. Most likely this is because you've added Microsoft.XLANGs.Pipelines to your project in order to execute a pipeline from within your orchestration. Go through your project references and ensure that any external assembly references that are in the GAC have Copy Local set to false.