Short version:
I am much, much more worried about their ability to take out our GPS and communications satellites than their launch of a ship, but I think military action is too far off to worry about. Our economic reliance scares me a lot more than their military technology.
Long Version...
China, more than any other nation, represents the greatest threat to the United States and our interests in the long term, however at this point those threats are much more economical than military. It is fortunate for us that our foreign debt (which they hold a considerable stake in) is measured in our own national currency, rather than that of the lending nation. That means if our currency takes a dive, so does their investment. It is in China's interest to maintain our currency's value, at least for the time being. While this may seem tangental, it's not. War dollars have to come from somewhere, and right now a lot of that "where" is in China.
When considering China as a threat, I would also not overlook the impact that globalization is going to have on that country. They are going to become more like us, and that means all of our problems, too. Right now, the vast majority of their people live on nothing, their social care is substandard, and their demand for energy resources is going to increase as those resources become more scarce, to name only a few of their problems. But, also give thought to their ingenuity as a Mr. Flash pointed out, they have a large population to draw on for solutions.
*Note these opinions represent those of the poster, they are not facts, so don't construe them as such.