Development

Part of it is hardware capability. The netflix app uses a lot of RAM and is pretty processor intensive, so it isn't so much the software as it is the hardware.

As well, by doing a limited release they can have, in a sense, a controlled test group, and also limit the strain on the server while the determine how much people will use it and how much bandwidth they (netflix) needs to support a full on mobile app like this.

That is likely a bigger reason why some phones that have the processing and RAM ability to support the app didn't get it, yet. There is also some software involved, but it is trivial given the developers have made a version of the app that will work on any device running 2.2 or greater.