Came across a nice presentation on EII today by Mark Madsen of Third Nature. It's called "Enterprise Information Integration Technology: Architectures, Uses and Abuses."
I highly recommend it to those looking to learn about EII and federation. It gives a very detailed view and advises on where and when to consider EII. I have to say, I couldn't have explained it better myself.
Here's a link to Mark's blog entry with the embedded presentation.