SQL Server Express is so limited that you really can't use it in production, and I've never had a case where Postgresql or Mysql couldn't handle the load (with enough architecture tweaks).

And you still need IIS. You can get that with Windows 7 Ultimate, but that's $319.95 for the retail price. And you pretty much can't get away without buying the retail version. We have deployed 8 HTTP servers, 2 MySql servers, another server with Varnish, 2 servers with Memcached and one for logging ... for one of the applications we maintain. Of course, we're using virtualization with Xen. But if we were to use Windows 7 Ultimate, then we'd have to shell out ... 13 * 319 = 4147 USD

On such a quantity you can probably get a discount, but let's be honest ... 4147 USD is almost the price we payed on the hardware, and I wouldn't run anything else than Windows Server in production which is a lot more expensive.

And if you're developing on top of PHP, Rails, Django, Mysql, Postgresql ... why on earth would you want to use a Windows server?