I took the liberty of copying a few paragraphs from James Joyce's book describing the causalist argument in Newcomb's Paradox. This is the best statement of the argument for taking both boxes that I have seen. I also included a short response of my own, which describes an alternate way of viewing the paradox based on multiverse models. It is at http://www.finney.org/~hal/Newcomb.html.
Hal Finney
