ok found how to do it. Just needed to use a rhinomocks stub on the request. This works for me.
public static RouteController GetRouteController(string useragent)
{
var controller = new RouteController();
var builder = new TestControllerBuilder();
builder.InitializeController(controller);
//mock out the useragent string
controller.HttpContext.Request.Stub(r => r.UserAgent).Return(useragent);
return controller;
}
Hope this helps someone else.