Serioulsy you people would ignore it?
Depends on the guy.
If its a builder I usually say hello as believe it or not, builders dont have much of a vocab, so a wolf whistle is just their way of saying hello.
If hes fit, I tend to say hello too and then they are stunned into silence which is pretty cool to watch.
If hes fugly I stare at him in disgust and tell him to naff off!