Emerson's Lee Neitzel, who was the technical lead for the Xi project, in this 15 minutes podcast provides a layman's overview of what Xi is, why it was developed, and how it is a common interface for the OPC Data Access (DA), OPC Alarms & Events (A&E), and OPC Historical Data Access (HDA). He describes the security and performance models.
After this introduction, the presentation gets more specific and provides more detail for software developers.