Using SL3, VS2008:
I checked in our silverlight solution (to sourcesafe), and other members of the team got the latest, and successfully compiled the solution.
When I run the app (exactly the same source code) on my machine, the property Application.Current.Host.Source.Host is set to "localhost". On the other team members' machines, it's an empty string (as is DnsSafeHost).
Why, and more importantly, what can I do about it?
Just a guess here but you would be running it in host web application, that is you have Web application in your solution that it is the set as the start up project. Hence when you start it the browser visits a http://localhost address.
However I suspect your other team members are running the silverlight application as the startup project, in this case the browser navigates via the file system to a test html file created in the debug folder. In this scenario there is no host.