I started working with the Unity Application Block just the other day. It is the Dependency Injection Block that has recently been introduced in the Enterprise Library Block 4.1. Personally speaking I found it much easier to understand and work with compared to the other standard DI tools like Castle Windsor and Structure map.
Some very useful links regarding Unity are :
http://www.codeplex.com/unity
Also you can check Scot ha's blog , whci contains a few number of podcasts which were very helpful to me.
Cheers...