Hysteria regarding these investment vehicles is nearing panic levels...which is when yours truly feels ethically bound to inject some skepticism and rationality into the discussion.
No, SWFs are NOT the biological equivalent of white blood cells, waiting to engage in financial phagocytosis upon the U.S. and hasten the demise of our way of life.
The reason is as follows: The liquid wealth (in terms of cash on hand as opposed to oil in the ground...the source of many of the more aggressive SWF's wealth) of these funds is not that ominous.
This is because SWFs consider interests in national companies as "assets" on their balance sheet. This is hardly a source of liquid capital that can be used to purchase, oh, say, the United States Senate or some other equally absurd consequence.
One need only look at the U.S. Treasury data tables to determine how much liquid wealth SWFs hold (the following is from the 3 major oil exporting countries, whose SWFs seem to elicit the most angst):
Treasury Securities: (Oil Exporters) $140.9
Foreign Exchange Reserves (Oil Exporters) $110.0
This is a far cry from "the natural enemies of the U.S. have 1.5 Trillion of our money and will buy everything" meme that currently infects us...it's no different from 1990 Japan which, if memory serves, promptly went bust shortly thereafter.