For a couple of years, Getty have had a relationship with Flickr where as they scan the millions of Flickr images, if Getty find something which fits with their requirements, they may invite the photographer to join the Getty collection (100,000 images have done so to-date). In return, the photographer would earn commission on any sales.
Now, a revised deal has been struck where Flickr members can nominate themselves for consideration. Predictably, there are two sides of the debate - those that thinks it's an easy, accessible way for photographers to monetise their art... and others who think naive photographers are being exploited (the commission is 30% for Rm and 20% for RF.
More details and an interesting Q&A with Getty available on BJP. How to join in the process is described on the Flickr blog.