Since there is not much documentation* on this yet on how to use OrchestrationEventStream (an EventStream that is in synch with Orchestration), here is a quick info on the howto:

This new feature is found in Microsoft.BizTalk.Bam.XLANGs assembly.  From you BizTalk project, you can write the following in your orchestration, e.g. Expression Editor (click on the appropriate shape in orchestration to launch this):

// Sample code for using OrchestrationEventStream from an orchestration, no need to new it:
Microsoft.BizTalk.Bam.EventObservation.OrchestrationEventStream.BeginActivity( "PurchaseOrder", "PO123");

Note: this only applies to Biztalk 2006.

* To use this in Biztalk 2004 SP1, you will need a hot fix, its kb article can be found here: http://www.kbalertz.com/Feedback_900083.aspx,