I thank James Chiang, Rick Edelson, Andrew Hamilton, Julia Lee and Mike Nowak for insightful discussions throughout the course of this work. CSR thanks support from Hubble Fellowship grant HF-01113.01-98A. This grant was awarded by the Space Telescope Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA under contract NAS 5-26555. CSR also thanks partial support from NASA under LTSA grant NAG5-6337. This work has made use of data obtained through the High Energy Astrophysics Science Archive Research Center (HEASARC) Online Service, provided by the NASA Goddard Space Flight Center.