Exactly. Its all good and well to say you should invade China and North Korea for the same reasons, but its not in the United States best interests to do this. The war in Iraq was very swift compared to most wars, and while lives were lost it was nothing like Vietnam or Korea.
Like others have said, China are a good ally and their labour market is very useful not only to the united states, but to the united states allies too. Not only that, but China also has a resonable defense force too (I believe they also have the largest land army in the world troop wise, although I may be wrong), and an invasion of China would be difficult and could potentially last for years, and could possibly drag america into suffering like the people of China are (possibly - I've never been to China so I wouldn't know).
In my opinion, the war on Iraq started because it was in the US's best interests. It was something that needed doing (removing saddam that is) but also risked a lot less lives than invading another country for the same reasons would have.
Don't get me wrong, I'm not saying the Americans are selfish and only care about themselves and are just trying to get rich. I'm saying they did what was best for their own people AS WELL as the world, and we should expect nothing more from any country, incluinging superpowers.
BTW, I like the way you put "australian prime minister" in brackets hehe, in case people didn't know who John Howard was. I'm an Aussie and I just found that funny when I read it, reminded me of how little role we play on the world stage.