Keyvan Nayyeri wrote a blog post a while ago about how to gather the remote IP from remote WCF connections. I wrote an extension method based on his code:

public static RemoteEndpointMessageProperty GetRemoteEndpoint(this OperationContext oc)
{
    if (oc == null)
        throw new NullReferenceException();

    object v;

    if (!oc.IncomingMessageProperties.TryGetValue(RemoteEndpointMessageProperty.Name, out v))
        return null;

    return v as RemoteEndpointMessageProperty;
}

Unfortunately it does not seem to work with the WCF implementation of Silverlight 3 :( .

Q: http://nayyeri.net/detect-client-ip-in-wcf-3-5

UPDATE: Switched to TryGetValue()