Good resources to learn loopy belief propagation
The following tech-report (a simplified version of an earlier paper by the same authors) on general BP (not only loopy-BP), but has fantastic insights.
It's a great place to start, if you already haven't. 

http://www.merl.com/publications/docs/TR2001-22.pdf

Another good place is this book:
Information, Physics, and Computation - by Mezard and Montanari.