To add to the conversation, here is an excellent article on when (if ever) to spay or neuter, including considerations of diseases that are reduced by spaying, as well as those that are increased by spaying, their incidence, and how to perform a cost/benefit analysis:
Determining the optimal age for gonadectomy of dogs and cats
It's probably behind a pay firewall for most people...
Edit: Free copy here!!!:
http://www.tc.umn.edu/~rootk001/gonadectomy.pdf