Development is one thing. What's the percentage that is of platforms for deployments? So I develop my rails app on the mac, if my production system is actually 20 machines and linux, that's a 20:1 ratio of servers deployments to development deployments.
Windows: 47.84% / Mac: 32.76% / Linux: 18.95% / FreeBSD: 0.07%
(iPhone is actually ahead of FreeBSD at 0.11% -- even though the site is not iPhone optimized at all)