Have we got general agreement in this thread that it is far more about attitude than attire? There really doesn't seem to be much disagreement there.
I've always gone the t shirt, shorts and thongs. There has never once been a bad sentiment about it, anything said, or any hint that it wasn't accepted. In fact, as numerous others have noted, it is basically the norm in most first class lounges these days.
Singlet and stubbies is probably another level, but again it has much more to do with the attitude and hygiene of the person than the attire. You would only notice it if they stunk or were being particularly rude. With women, shorter shorts or singlets obviously wouldn't be questioned. The only conceivable issues lie with the observer, and I'm quite certain will only ever be triggered when other factors simultaneously don't meet their satisfaction.