I had doubts that this rumour would be true. Sega and Nintendo have a long history of rivalry. I know that on occasions they have worked together on projects (F-Zero GX is one example) but allowing Sonic to feature in Nintendo's most money consuming franchise is a big mistake from both companies. Allowing them to star in spin-offs together is acceptable but allowing them to star in main games should be avoided. If they want this to happen create Mario & Sonic's Adventure.
@GameGuy100 I would like to challenge you in Mario Kart Wii, I am a pro when it comes to the game (I also have everything in Mario Kart Wii that you can possibly get)