Remember the reaction to the Bush doctrine of pre-emptive strikes and the concern that raised. Sure, by one argument a level headed government may have a right to attack a location or country if they believe an attack from them is imminent (and I don't want to turn this into a 'was Iraq a threat' debate so leave that part well away from this) but the fear was that a precedent was set with little or no guidelines, rules or boundaries as to what constitutes a threat and an agreeable reasoning for launching such an attack. It basically sets up a situation, and this is in part where the failures of Iraq come into the argument (even if you don't think there are failures, there certainly are widely perceived failures that can be used effectively in an argument) because who says what is a threat and what isn't, what evidence or argument is needed, what role does the UN and international community play etc? Apparently no-one knows, so the fear has always been that once the US set that precedent, someone else will jump in and then point the finger back at the US. Lets say the US did take action against Iran. China/Russia may be pissed off by that, but unable to retaliate directly as Sting says. China however, for example, may point to the US supplied missiles etc on Taiwan and say "Threat! Pre-emptive strike necessary as per the US lead and as per the evidence of an aggressive US in recent history!!" and off they go. Messy, no? So while the reality is that China or Russia or whoever depending on the situation could kick up whatever stink they want over Iran, any retaliation over it will probably come elsewhere and will be more to spite the US than anything else. Cool with that Sting? It would be Bush's rules they're playing with. Ridiculously hyperthetical of course, but it's the worry people have or had with the US pre-emptive doctrine, and with how the lead up to the Iraq war went down. China might well be able to build a stronger case for Taiwan as a threat than the US could against Iraq or Iran? The US arming them to the teeth, the US conducting naval training right there, all time. The US happily launching invasions on countries left, right and centre. China may find this threatening? Strike first! The rules now say we can! Not trying to divert the argument, but that's where the issue could lie, not so much in the US and Russia/China clashing directly over Iran, but more a freakin' mess everywhere. Again, not of it is even close to likely, but it's not like 'worst case scenario' type things haven't happened before, and this is the kind of fire that could be played with.
And as an additional scenario type question for Sting - ignoring the Iranian question here, but what if out of nowhere tomorrow, China starts employing the exact same game plan the US used over Iraq, but over Taiwan? Claimed it as an imminent threat that they have a right to get to first? As a starting point, what if China demanded that Taiwan be disarmed?