The BizTalk Server 2010 Feature Pack is an add-on to BizTalk Server 2010 that allows BizTalk functionality to be invoked from Windows Azure’s Service Bus. The feature pack specifically includes “BizTalk Server 2010 AppFabric Connect for Services”, the feature that allows you to do this.
You can read more about how it works on the BizTalk Server Team Blog. Great feature!