There are tons of startups using the Microsoft stack... Just because they don't live on HN or don't appear on Techcrunch does not mean they don't exist.
Microsoft even has BizSpark - a network to provide resources to startups (we are one). All you can eat Microsoft software via MSDN.
"Price" is no longer an issue and Microsoft has made a point of building a solid startup ecosystem around their stack.
That "Price is no longer an issue" is a falacy. You are ignoring the cost of patching and rebooting Windows boxes and the cost of the redundant infrastructure required to accommodate the longer downtime Windows patches require.
And price is no issue only if you fail. If you succeed and have to deploy dozens of servers, you will quickly feel the it in your wallet. You can't grow horizontally as cheaply as you can with any free (even free-as-in-beer) solution.
I have better automation facilities on Unix-like servers than I can have on Windows. Configuration replication is rather easy. My colleagues here manage 1000+ Linux servers with a homemade solution based on Puppet and Fabric and can deploy a server from power-up to ready-to-go in a couple minutes.
If you can read Portuguese, they did a very good presentation at FISL in Brazil a couple days back.
Microsoft even has BizSpark - a network to provide resources to startups (we are one). All you can eat Microsoft software via MSDN.
"Price" is no longer an issue and Microsoft has made a point of building a solid startup ecosystem around their stack.