I'm not sure if this was a change in IIS 7 or if someone before me actively removed X-Server from the HTTP response headers but I need to be able to tell which server in the farm served the current page. I don't see X-Server in the custom variables section of IIS but I'm not sure if it was a custom variable in the first place or if it's in the defaults and it's been disabled via web.config. How do I get my headers back?
You can use custom headers (http://www.iis.net/ConfigReference/system.webServer/httpProtocol/customHeaders) to add any header you like.