Okay, time to clear things up. Super Mario 128 isn't a game, and probably never will be. Super Mario 128 is more of a concept, a web of ideas if you will. It's basically a project where Shigeru tries innovative stuff for future Mario games.
"Well, Mario 128 was a collection of different experiments. Super Mario Galaxy takes a lot of the experiments from Mario 128 and implements them in the game. Because of that it's partly 128 but there are also elements that existed in Mario 128. I don't know when we'll be able to bring those out." -Shigeru Miyamoto
He basically said a lot of the stuff from 128 is in Super Mario Galaxy.
I hope this clears things up! ^_^