U.S. (and British) nuclear planners responded to the Soviet deployment of a limited missile defense system with enormous firepower. The large number of nuclear weapons that were assigned to overwhelm the Soviet ABM system and the substantial technical efforts the U.S. undertook to defeat it provide chilling examples of the attention missile defense systems attract from hostile nuclear planners. It is a history that fundamentally contradicts the portrayal of missile defenses as non-offensive, threatening no one. Ballistic missile defense systems threaten secured retaliation, and for smaller powers, deterrence itself.
Missile defense systems also indirectly threaten populations. The Soviet ABM system was intended to protect Moscow against nuclear attacks, but rather than shielding the capital from nuclear peril, the system in fact had the opposite effect of attracting nuclear warheads. Many other facilities would have been targeted in addition to the ABM system, including political and military leadership targets. "We must have targeted Moscow with 400 weapons," a former Stratcom commander has stated. 
What is the relevance of this today? One could argue that all of this occurred during the Cold War, that U.S.-Soviet/Russian strategic competition is over, and that smaller nuclear powers do not have enough nuclear weapons to overwhelm missile defense systems. That may or may not be so. But at the superpower level, the action-reaction momentum seems to continue.
The United States apparently still targets the Moscow ABM system, and Russia appears to have begun adjusting its own forces to a future U.S. missile defense. The Bush administration's claim that its system will not be of concern to Russia may be true in a hypothetical Russian first-strike scenario with hundreds of missiles. But Russian planners are likely to be much more concerned with the effect on their surviving retaliatory capability after a hypothetical U.S. first strike has reduced the number of operational missiles. This will almost certainly drive new modernization efforts, newfound U.S.-Russian partnership or not.
For China, the situation is drastically different. The credibility of its nuclear retaliatory deterrent will be fundamentally challenged by a U.S. missile defense system. Ironically, the situation is similar to that in the late 1960s, when China was the "rogue" state used as the justification to build the first limited U.S. missile defense system. Back then, a system with 100 interceptors, the same capacity planned by the Bush administration today, was thought to be capable of reducing U.S. fatalities from a Chinese attack to "possibly zero, if the number [of Chinese missiles] does not reach 25."  China today has approximately 20 ICBMs capable of hitting the U.S. mainland.
The NMD proposal is gravely destabilising. In 1968 the Soviets built a partial ABM system around Moscow. The US responded by altering its nuclear attack plans to overwhelm the ABM defences. Russia has already announced development of a new missile capable of passing the any US NMD system.
START II banned ABM systems because all they achieve is faster growth in attack systems. That was signed by the notorious appeaser and soft-on-communism George H W Bush. It was unilaterally abrogated by George W Bush. The US has not given reasons why NMD will not aggravate the arms race beyond the usual stuff about being exempt from history and rogue states. Russia has now announced a new missile which renders all missile-defense systems "useless." We should not be surprised. This is exactly how the US responded in 1968.
More on the rogue states argument later.
Australia should not join this system.