Public barred from the Public Gallery

Loud has been the uproar over the decision to bar the public from the Public Gallery in Parliament House during President Bush’s visit.
Whilst my mind is not made up on the issue, one must consider the security matters weighed against the curtailing of freedoms before automatically decrying the measure as another fall down the slippery slope towards a Police State.

Obviously President Bush is one of the biggest if not THE biggest target for political assassination in the world. In fact, if you were to do a poll of everyone in the world with access to explosives and other weapons and who they would most like to use said weapons on, you would most likely find that George would appear very prominently on that list if not at the top. This is most likely true for any American President however.
The problem here is that if any of the above whackos were to get past the security at Parliament House and detonate their explosives in a suicidal attack against Bush, not only would they have taken him down but depending on the size of the bomb they would have wiped out a large portion of the Australian Parliament.
The loss of our Prime Minister, Opposition Leader and all of their successors (whilst comical to talk about now) would be a devastating event with far reaching ramifications for all of us.
Imagine if you will that this did in fact happen. If Parliament House lay in ruin and all of our politicians killed (once again while comical to think about now) would we still think that not letting the public into the gallery was so ludicrously over the top?
Of course the likelihood of someone slipping past Parliament House security with enough explosives strapped to them to do this would be extremely small. Possibly about as small as the chance that someone could hijack two 737 jets and fly them into the World Trade Centre?