I agree, it is a lovely piece, Gatsby, and I really enjoyed Williams' arrangement. The original melody, or song, by Joseph Brackett has been incorporated or used by many other composers, most notably Copland.
http://en.wikipedia.org/wiki/Simple_GiftsIt is often performed as a vocal with Joseph Brackett's original lyrics.
http://www.youtube.com/watch?v=HILWa4U-3Ac&feature=relatedFrom what I understand, John Williams often "borrows" from other composers and incorporates them into his own works. This particular composition seemed just perfect for the inauguration, particularly because it had historical echoes of traditional American music.