What Mario game is Geoff Keighley teasing?

Geoff Keighley is teasing some sort of Mario-related game for the holidays. Check out this mysterious message left on his Twitter account…

“Someone introduces himself as ‘It’s-a-me, Mario!’ in a big holiday game, and it’s not New Super Mario Bros Wii.”

Well, this is pretty odd! I honestly have no idea what Keighley is talking about, unless it’s that Mario arcade game. Charles Martinet said a few weeks back that he was recording voice work for a Mario game that isn’t New Super Mario Bros. Wii or Galaxy 2, but is Nintendo really preparing another title for the holidays? And if this was something obvious, wouldn’t Keighley have elaborated? Perhaps it’s a WiiWare title? I hope we find out soon!