Interesting how YY mentioned bout this concept. If I'm not mistaken, I watched a movie with the same title two years ago. Can't really remember the storyline, but the message that was being put through was definitely strong.
I just found out that there was a book written bout it too. Think that the movie was based on that novel.
Anyways, if you'd like to know more bout the 'Pay It Forward' concept, feel free to check out http://payitforward.warnerbros.com/Pay_It_Forward/.
Spread the word and start paying it forward whenever you have the chance.