The United States is/are

[Frank Abate]

Paul et al.

I heard Allen Walker Read give a wonderful paper (like all his papers) on
"United States" some years back at an ANS or ADS meeting.  He was focusing
more on the inhabitant-adjectival uses over the years, but may well have
touched on the singular/plural thing.  I don't know if the paper was
published, but I'd check the index for "Names", the journal of ANS, and also
American Speech.

Frank Abate