Lynn Sislo had a small rant the other day about uncredited musical rip-offs -- how Pachelbel's famous Canon in D has now, seemingly, morphed into something called "Christmas Canon", with no mention at all of poor Pachelbel. (My favorite version of the Canon, by the way, is the Canadian Brass's version -- although I deeply pity that pour soul of a tuba player.)
Anyway, this kind of thing -- stealing melodies for newer works, even popular song -- has been around for years. Two movie scenes leap to mind: First, the wonderful Mr. Holland's Opus (which I seem to cite weekly now) has a scene where Mr. Holland decides to take a different approach to trying to get his students interested in music. He starts playing a pop tune called "Lover's Concerto", gets them excited, and then informs them that it's really a Bach minuet. And in the musical Anchors Aweigh! there is a scene where Dimaggio (Frank Sinatra) comes across a pianist (Jose Iturbi, playing himself -- it's a long story) who is playing the famous melody from Tchaikovsky's Piano Concerto #1, but to him it's a pop song that he sings with gusto.
Composers always borrow. It's what they do. (And if you want to see some freewheeling discussion of musical "borrowing", just go to any film music discussion forum and ask about James Horner. You'll get an earful.)