That's true and I think Stack Overflow actually did a lot for making it more acceptable in startupland. And for good reason - if it's good enough for them...
The Linux advantage really starts to show when you enter the dozen+ server range. Licenses add up quickly and planning gets more and more complicated.
If you run a small group of machines and you know what you are doing, Windows could be just fine for you. The worst that can happen is you succeed and need to migrate your software to another platform.
edit: actually the worst that could happen is that you hit a technical problem the closed nature of Windows prevents you from solving and you have to port before you become profitable.
Be careful here though ...
> I know better than to expect to converse about our MS technology with the HN crowd, though.
Lots of positive posts, even in this thread!