This game was confirmed by Craig at IGN pocket in the latest mailbag. He said that he was unable to talk about it until another publication (EGM, I guess) mentioned it. That's all he said.
Maybe it will be an overhead game, like Perfect Dark was on GBC...pretty exciting, anyway, I'm a bit shocked that MS is allowing the brand to be published on the GBA. (And I agree that the DS would be a smarter place for it, but Microsoft is obviously trying to make lots of money, which it will on the Gameboy, but maybe not on DS.)