Jim Baumbach of Newsday points to a blog post written by Mark Mussina, Mike Mussina's brother. In it, he says:

Before I get to playoff match-ups, I want to say thank you for all the kind words regarding Mike’s season, and that I really don’t know what he is going to do next year. I think he was mentally prepared to retire, but after doing so well, and knowing how close the Hall of Fame is (some people think he’s in already, but who knows) I think it’s going to be hard to walk away. At this point, that’s all I will say about Mike.