I've also noticed the strong resemblance they have, especially when watching old Robert Redford films, such as The Way We Were.
They should do a film where Robert Redford plays the father and Brad Pitt plays his son, or maybe Pitt could play the 'younger version' of Redford in a flashback. (If of course these things haven't already been done. Maybe they have, I haven't been to the cinema for a while!!)
Of course it's well-known that they're not related, but when Googling this question I found that another hubber has also posed this question (as well as many other people who have also seen the spooky similarity!).
Take a look at http://hubpages.com/entertainment/Robert-Redford-a... for some cool comparison photos, and the comments at the bottom of the page.