Now that it has been a while BizTalk has been around and folks have started getting experimental with it, I too thought of exercising my lazy brain cells on how to apply configurable design over orchestrations.
The need for this could be prominent in scenarios where things change frequently e.g. different sorts of requests are added for the same kind of processing, flow of orchestration is to be altered (w/o bringing down the orch). Now mostly the maintenance folks are extremely capable of .NET development yet they would prefer to avoid BizTalk development for obvious reasons (assume the most positive sense here :)). So the question remains on how to avoid BizTalk development in case of a change request etc and yet be able to alter orchestrations.
So the approach here from a bird's eye view is to pass on XmlDocuments to a Master orchestration and then let it pick up components/orchestrations to attain the effect of a workflow from a configuration file. In this case, even basics such as validation become configurable since the first step the Master Orchestration could do is to pass the incoming document to a Request Identifier of some sort (based on XPath may be) and then from the configuration file again an apporpriate validtor could be invoked; now please do get carried away and think of what not could you do from this point on based on the configuration e.g. process sequence, invocation of other orchestrations with atomic/indivisible steps, invoking web services, compensation logic.
In synopsis, the basic idea is to prevent the 'hard-coding' of the orchestration itself and yet leverage the BizTalk platform to process requests.