However, I am fairly well wed to Linux/OS X, and don't see myself being productive with Windows (whenever I used Windows, the first application I'd install is Cygwin), nor (having never done Windows Systems Engineering) do I know how to find a great SRE/operations engineer for a Windows system (or how to train a great Linux SRE to be a great Windows SRE). Nor can I legally setup a small cluster of Windows Server machines in my apartment (or on Ec2) to prototype the application _before_ I officially start a company (and qualify for bizpark) without paying extra costs. Ditto for setting up a developer desktop with MS Visual Studio.
I am watching Mono seriously, but for now I'd stick to C++, Erlang/OTP, OCaml, and Java/Scala as my choices (with Python or Ruby for scripting) were I to start a new project.
Addendum:
Microsoft: if you're reading this, buy Miguel De Icaza's startup. Make Linux and OS X a first class side deployment platform. Market SQL Server (which, from what I recall is superior to it's primary competitor, which is Oracle) as a way to "upsell" customers using Mono on Linux to use Windows Server (if it makes business sense for them).