To get the identity of the calling user of your web method, you can use something like this:
string name = OperationContext.Current.ServiceSecurityContext.WindowsIdentity.Name;
Constraint: your service contract must have a session. In WSSF you can change this in the Service Contract Model on the ServiceContract element.
Don’t forget on the endpoint (Host Explorer) to add an enpoint with a wsHttpBinding Binding Type.