I am not a lawyer, but as I understand it, Steamboat Willie is entering the public domain, not Mickey Mouse. Mickey Mouse is a trademark, and trademarks don't expire.
I'm not entirely sure what that means here, but I've got a strong suspicion that the guy would still not be able to get away with a Mickey Mouse comic book.
There was a law school article submitted a few days ago that provides a very good roundup:
Mickey mouse as a concept will enter public domain. Although with a trademark, it will be hard to do a Mickey mouse animation.
See Sherlock Holmes. Some, but not all of Sherlock Holmes is on public domain.
Also note that trademarks expire, they have to be renewed. In addition, you can lose a trademark. See Kleenex, escalator, zipper, bandaid, etc.
This is one of those "wont believe until I see it"
Congress is already on winter break. It's going to happen.