I've just had two titles bumped from my Netflix Queue to my Netflix Saved section, suddenly, and with no explanation. One is Pedro Almodovar's All About My Mother, and the other is Henry - Portrait of a Serial Killer (20th Anniversary). The titles have gone OOP [out of print]. All About My Mother is on Amazon.com for $20-49.95 and Henry - Portrait of a Serial Killer (20th Anniversary) is there for $16-22.
What does it mean for something to go out of print? Well, books and DVDs are manufactured, just like shoes and toilet paper. When a publisher or movie distributer decides to sell copies, they hire a printer to do a job for them, known as a print run. They order a certain number of discs or books, based on how many they think they can sell, based on pre-orders from stores and customers. A run can be hundreds or thousands of copies. If they succeed in selling all of their copies, they are done. The Book or DVD is now "out of print." You are not aware of this until all the copies are gone from the stores.
Netflix buys many copies. Each copy lasts through maybe a dozen rental cycles before it wears out. When all of Netflix's copies wear out, they order more from the distributor. If there is a second print run or third print run, we're in luck. However, if the distributor doesn't do another print run, Netflix has no control over that. Netflix isn't about to go to eBay or Amazon or Blockbuster to buy more copies, because that would not be cost-effective.
Going out of print doesn't mean that there are no more copies anywhere. It means that the manufacturer has sold all the copies that were made. You can still find copies at your local retail or rental store, or online at other stores or rental outlets, but the price might go up as collectors and speculators take advantage of the shortage.