I loved the book by Catherine Ryan Hyde and the movie starring Kevin Spacey. In the book and the movie, the idea is described as an obligation to do three good deeds for others in repayment of a good deed that one receives. But the idea of paying it forward is not a new idea.
In ancient Athens (317 B.C.), the popular play "Dyskolos" by Menander used the concept as a key plot element. The text disappeared and was not re-published until 1957.
In April of 1784, Benjamin Franklin lent a sum of money to Benjamin Webb, and told him pay him the sum when he could by giving it to someone else who was needy. Franklin hoped it would go through many hands before there was some "Knave that will stop it's Progress. This is a trick of mine for doing a deal of good with little money."
Ralph Waldo Emerson in his essay "Compensation" wrote "the benefit we receive must be rendered again, line for line, deed for deed, cent to cent, to somebody."
In 1944, an anonymous spokesman for Alcholics Anonymous said in the Christian Science Monitor, "You can't pay anyone back for what has happened to you, so you try to find someone you can pay it forward."
And so it goes. A variation of the Golden Rule? Perhaps - but that rule taken a bit further!
And by the way, there is a Pay It Forward Foundation. You can check it out @http://www.payitforwardfoundation.org/.