Only the first episode of season 2 is available to watch instantly. It's just a promotional thing that Netflix and Showtime do every once in awhile. They did the same thing with Dexter.
Note that Showtime has elected to digitize the buttocks and boobies that would otherwise be seen in the Tudors episodes, but only for the promotional preview. You're going to have to pay to see those!
Thanks for the replies everyone! It makes sense now. I have Tudors, Season 1 coming in the next day or so.. I was just curious about the insta-watch thing.