Getting used to Internet Explorer, forgot to put my name on my question about "A Guest of Honor." Don't have to fill in the name with Netscape.
I have heard the story about "A Real Slow Drag," but Gershwin wrote too much good music on his own; I tend to doubt it. Certain simple musical lines can appear coincidentally. I don't know how you would go about "proving" it one way ot the other.