Creating a WCF Client in Visual Studio 2010, I struggled with this error.  I was running as the currently logged on user and the service was as well so this made no sense to me.  I found that this entry in the client app.config was the culprit:

<userPrincipalName value="MYDOMAIN\MyUsername" />

Simply deleting these 3 lines allowed me to test the client.

Let me know if this helped you!