No, it's 100% streaming. $8 a month is the streaming only subscription option for Netflix, more than Amazon, though they have a lot more titles to stream. They touted 12,000 titles available to stream back in '09, and I think it's higher now. I'm just wondering which of Amazon's 5,000 titles overlap with Netflix's 12,000+ titles.
Netflix doesn't actually HAVE rentals as far as the streaming part goes. You stream as many titles as you want for the monthly fee, you don't "rent" on a title by title basis. I like it because I'm much more likely to watch something I don't know when it costs me nothing extra than if I had to pay to rent it on it's own.
BTW, the Roku is great, and they are cheap. They do Netflix, Amazon VoD, MLB.tv (with sub obviously), Hulu (with Plus sub), Pandora, and they have a bunch of other channels like video podcasts and the like.
Edit: I see current estimates that Netflix's streaming catalog is around 20,000 titles now, but I don't now the veracity of that claim.