The man who has voiced Mario since Super Mario 64, Charles Martinet, has let loose that Nintendo has a new mario game planned already. He promises that it's not New Super Mario Brothers Wii or Super Mario Galaxies 2.... So what is being planned?
First thoughts would be that it would be a handheld title, but with the voice of Mario having hold of this information, it's likely he'll be voicing for it, pointing against that notion. It's also unlikely that it will be a downloadable title, considering how massive the Mario games tend to be. I may be wrong but chances are it's going to be a wii title. They definatly won't release it within a year or two of Super Mario Galaxies if it's a wii title, so that would mean a release close to 2012. Maybe Nintendo will have a new console ready to showcase at that time.