The Teflon Don at Acute Politics has a good post up explaining the ISF. He's a Marine who is stationed in al Anbar province. He had a front row seat for the Anbar Awakening and is there watching the progress day to day now.
I knew the Iraqi version of "Neighborhood watch" was different from ours, but now I know how it ties in with the ISF. It's a short read so check it out.