A database of Wii U eShop entries on WiiUBrew has listed Super Mario Galaxy among the games that could very well be coming to Wii U in the near future. The game has been given a title ID of 00050000-101BFF00 in the system with it being tagged for a European release.
The site is ran by a known researcher and data miner zhowel, who has been known to correct in most cases. So if this really is true, expect an announcement shortly. Keep in mind that this is still a RUMOR and nothing has been confirmed at this time.