I'm going to have to side with gweedo as well... ASP.NET is horrible for scaling. Not technically, although the things POF.com have had to do to make it run well are pretty damning, but financially. A startup could really put to better use the $800 spent on each Windows Server license, not to mention the $2500+ for a MSSQL server (1 core!!!) license. Even their bizspark stuff has costs looming in the future. Also, for a Bitcoin web site you don't want to touch Azure for security reasons (at least not for the backend.)
ASP.NET is precompiled, so it will always be faster than php.
You don't need to spend any money up front when developing in .NET technologies. You can use encryption on Azure, and scale as you need to.
http://www.windowsazure.com/en-us/
Who is going to take the advice of a man who's icons is a fat nibble licker.
Thanks. but no thanks.