Here's a breakdown based on referrals from YC to a short essay I wrote about a month ago comparing Bing Travel and Kayak. There were 2382 uniques from YC over two days to that page:
45.5% Windows
37.6% Mac
15.1% Linux
Comparing that with an e-commerce site I work with which has a less geek-centric audience, during the same two days: 84.94% Windows
12.46% Mac
1.55% Linux