I had the same question come up last night during dinner and didn't know the answer. I searched the internet and found the following internet site:

http://www.ipl.org/div/mushist/twen/american.htm

And, if you read the following, I believe that "Someone to Watch Over Me" comes from the Folk Opera "Porgy & Bess". Hope this helps.

Born: Brooklyn, N.Y., September 26, 1898
Died: Beverly Hills, Cal., July 11, 1937
The first American composer to successfully combine popular and serious styles, Gershwin had his first success at the age of nineteen when he wrote the song "Swanee." 1924 saw the premiere of his first successful effort in jazz-inspired "serious" music, the now famous Rhapsody in Blue. Throughout his short career, Gershwin wrote both popular songs and musical comedies, as well as concert works, such as the Concerto in F for piano. To both he brought the consummate skill and style of a trained musician, while at the same time infusing both with the elements of jazz, blues, and Latin dance-rhythms that were prevalent during the periodin which he lived. One of the best of his concert works is the symphonic poem An American in Paris. Premiered on Broadway, Gershwin's folk-opera Porgy and Bess has now been staged by several major opera companies in the world, including the Metropolitan Opera in New York City. Of Gershwin's most popular songs (including "Someone to Watch Over Me," "Embraceable You," "I Got Rhythm," and "But Not for Me"), many come from this opera, including the well-known "Summertime".