I came across an error when configuring claims-based authentication for Microsoft Dynamics CRM 2011. The error, Service is Unavailable was returned when browsing to the CRM metadata URL.
In this installation, something was wrong with URL rewriting. Following the steps in this article resolved the issue:
http://blogs.msdn.com/b/emeadcrmsupport/archive/2011/05/13/we-receive-http-errors-while-accessing-the-crm-federationmetadata-url.aspx