Very late additional answer:
As others have noted, today "the United States of America" is just about always treated as singular.
However, you will occasionally hear someone refer to "these United States of America", or say "the United States are ...". Sometimes this is done for political reasons: a hot subject of debate in the US is what the relative power of the federal government versus the state governments should be, and some who believe in greater autonomy for state governments will deliberately say "these United States" to emphasize the idea that there are, or should be, multiple sovereign state governments. More often it's because people want to sound "old timey".
Readers Digest magazine used to have -- I'm not sure if they still do, I haven't seen a copy in a while -- but they used to have a feature with jokes about American life that was called, "Life in These United States".