Both Amazon and Netflix are about to collectively reach a much larger audience, with the introduction of Amazon's Video on Demand service to the Netflix Roku. The tiny media player that Netflix sells, originally designed solely for their streaming service, has been somewhat displaced by other hardware that the video rental company also opened up their service to. Amazon's VoD introduction brings a new avenue for profit for them, as well as offering existing Roku owners a lot of new content for a price, of course.
The big difference between the two is that Netflix's service is subscription based, whereas Amazon's is on-demand, where you pay per video you watch. Given the different nature of the services, it seems logical that they will blend together on the Roku. If Netflix doesn't have what you want, Amazon's service might with no need to sign up for something extra. It definitely gives the Roku another advantage over other hardware devices, from Blu-ray players to the Xbox 360, which also can stream Netflix video.