1. J. Balthrop and S. Forrest are in the Department of Computer Science, University of New Mexico, Albuquerque, NM 87131, USA
2. S. Forrest is also at the Santa Fe Institute, Santa Fe, NM 87505, USA.
3. M. E. J. Newman is in the Department of Physics and Center for the Study of Complex Systems, University of Michigan, Ann Arbor, MI 48109, USA.
4. M. M. Williamson is at HP Laboratories Bristol, Filton Road, Stoke Gifford, Bristol BS34 8QZ, UK.