Amazon unveiled its new “video on demand” service today, just three days after Netflix and Microsoft announced that they’d make 10,000 movies available for streaming via the Xbox 360, confirming that an all-out war is underway for the next big shift in movie/TV viewing habits.

Amazon’s program will reportedly allow roughly 40,000 movies and TV shows to be streamed over the Internet. A third recent headline came from Sony, whose PS3 system enables movie rentals and downloads as well.

Given the difference in scale, the Amazon service clearly aims to become the gorilla in this category. There appear to be other advantages over the Microsoft/Netflix service, which is called “Watch Instantly,” in that Amazon will offer new releases in addition to the older films available on Netflix.

Stay tuned for further developments in this slice of the media industry. Everything is changing too fast to draw any conclusions — yet.