For the United States, Webster's Collegiate Dictionary says: "plural but singular or plural in construction." Oxford says: "In later use, frequently construed as a singular." The Encyclopaedia Britannica used the singular in its 1888 edition.

Wolfgang, you are correct in that most countries ending in 's' would be plural in English (but Wales is).

Some of the Latin American countries formerly used "Estados Unidos" (United States) in their names since they also were composed of "states," but I think that usage has disappeared. The United States of Colombia was formerly the correct appelation of that country.

USA? US may be singular or plural depending on context-construction- but United States of America nearly always is singular in American usage.