In Enterprise Library 5 Unity performs most of the heavy lifting for the Policy Injection Application Block. (Unity is a dependency injection container leveraged by Enterprise Library.)
From the Developer's Guide:
[The Policy Injection Application Block] is provided mainly for backwards compatibility with existing applications.
In version 5.0 of Enterprise Library, the recommended approach for implementing policy injection is through the Unity interception mechanism. This supports several different techniques for implementing interception, including the creation of derived
classes rather than remoting proxies, and it has much less impact on application performance.
this answer on Stack Overflow Policy Injection is implemented as a Unity Interception Behavior.
Unity interception is the official recommendation but Policy Injection does give you access to the GUI Configuration Tool (which is nice). However, it looks like Policy Injection could be a candidate for removal from a future version of Enterprise
Library (it's already relegated to the Appendix of the Developer's Guide and most of the Call Handlers have been moved to other assemblies).
Enterprise Library support engineer