I’ve just been pointed to an interview with Shigeru Miyamoto by Wired, an excerpt of which reads:
WN: This is the first traditional side-scrolling Mario game Nintendo has made since 1991. Why wait this long to do a new Mario game in the original style?
Miyamoto: Because we’d spent all this time working on 3-D Mario games. And of course, when we had Rare developing the side-scrolling Donkey Kong Country games, we were able to rely on them, which kept us from coming back to side-scrollers for a long time.
WN: But there’s still no sign of the long-rumored Mario 128 for GameCube.
Miyamoto: It’s still floating around. We’re searching for that fundamental idea that’s going to drive the next 3-D Mario game. But we’re not sure when that’s going to jump out at us. We’re doing lots of tests with small groups.
WN: If that’s the case — if the design process is still at the point where you’re doing experiments and tests — is it even possible that Mario 128 could come out on GameCube at this point? Or is it definitely a Revolution title?
Miyamoto: We think we want it on Revolution.
WN: So, there will be no new GameCube Mario platform game.
Miyamoto: Right. The Mario team can’t create too many games at the same time, so they’re concentrating on the Revolution.
A summary of which was provided on a game forum thus:
Kohler: So, Mario 128. Where the fuck is he?
Miyamoto: We’re, uh, still running tests.
Kohler: So you’re telling me nothing actually exists and you’re just jerking us around.
Hate to say I told you so…