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http://www.microsoft.com/bizspark/about/Graduation.aspx
Through the grapevine I hear the "review" process (for more free licenses at graduation) is extraordinarily accommodating, though I'd guess if you're spinning up a box per customer or something insane you'd get denied.
Lots of people get this wrong, I'm surprised Microsoft hasn't carved this correction into a mountain somewhere...
*Stack Exchange is running 2 + failover ( http://blog.serverfault.com/2011/09/30/the-stack-exchange-ar... ), and was running on 1 not too terribly long ago.
Only thing i really miss from pro is plugins, version control-integration and code contracts, the last being a thing that probably justifies a price tag anyway.
Now their WinForms ability might be great, but there are few start ups out there at are actually doing anything with desktop development.