As for any MGS game being made for the Wii, allow me to quote an interview from CVG.
CVG: What are your impressions of Nintendo Wii and how did Snake appear in Smash Brothers?
Kojima: I’ve played Tennis, Zelda and Mario on Wii. I find Tennis very interesting. I played the early version but the E3 version is far more advanced. As a game designer, I’m very interested in creating something for the Wii. I’d like to run away from MGS 4 creation and create something for the Wii but unfortunately, I don’t have anything that I can announce at the moment. I have lots of ideas for Wii but I have a heavy schedule with MGS 4 and Metal Gear Online. Concerning Snake’s appearance on Wii in Smash Brothers, I’m friends with the designer of the game and I thought it would be nice if you could play as Snake - when I heard about the new game last year, I contacted the designer and we started working together.