Y Combinator is (we hope) visited mostly by hackers. The proportions of OSes are: Windows 66.4%, Macintosh 18.8%, Linux 11.4%, and FreeBSD 1.5%. The Mac number is a big change from what it would have been five years ago.
I think even this stat is misleading as many of the readers probably still work for an organization that determines their workstation environment. I think what David is talking about is what do you use your hard earned money to buy and work on when it's completely your choice.
I believe that this is relevant as well: http://www.paulgraham.com/goodart.html. Even though we are talking about tools and not art there is clearly a better one. If you think it's just a matter of opinion, spend several months developing on Windows then Linux then OS X and get back to me.
When I worked as a sysadmin in a company using Windows NT 4 then Windows 2000 I must admit that I didn't understand why people used mac's either. I thought is was just style or monopolizing education and the creative market. It wasn't the intel processors that changed my mind. It was --
"The reason, of course, is OS X. Powerbooks are beautifully designed and run FreeBSD. What more do you need to know?" -- Paul Graham
I was already using FreeBSD on all my servers and it dawned on me that a great UI on top of BSD was the way for me to stop messing with linux and focus on writing code.
After reading what I've just written it dawns on me that I really don't care what others use. I guess I just don't like reading what I feel are incorrect statements by people that have obviously not worked extensively on all 3 environments.
(http://www.paulgraham.com/mac.html)
The percentage of Mac-using Y Combinator founders appears to be even higher. Actually the majority, I think.