We thank Cole Miller and Eve Ostriker for stimulating conversations conducted throughout the course of this work. We gratefully acknowledge support from the National Science Foundation under grant AST0205990 (CSR), the Chandra Cycle-5 Theory & Modelling program under grant TM4-5007X, and the Royal Society (ACF). All simulations presented in this paper were performed on the Beowulf cluster in the Department of Astronomy at the University of Maryland, kindly administered by Derek Richardson.