Many people including myself are running into this problem while sharing and running WCF samples using the .NET Framwork 3.0 RC1.  If you receive the following...

Exception
[System.ServiceModel.Security.SecurityNegotiationException] {"SOAP security negotiation with 'http://localhost/OrderService/OrderService.svc' for target 'http://localhost/OrderService/OrderService.svc' failed. See inner exception for more details."}

Inner Exception
[System.ComponentModel.Win32Exception] {"The Security Support Provider Interface (SSPI) negotiation failed."}

Try changing <userPrincipalName value="MACHINENAME\ASPNET" /> to <servicePrincipalName value="host/localhost" /> in the client config file.