[jcranmer]

Trademarks are about protecting the use of "marks" (i.e., logos, names, etc.) for use in "trade". So having a trademark on Mickey Mouse means that you can't use Mickey Mouse to advertise or promote your product, and this can probably construed broadly (e.g., sample screenshots of a game, packaging material). Even if it can't Disney is not known for being shy on litigation, and it's totally possible for them to sue you even if they have only a slim chance of victory. Using Mickey Mouse as a cameo would be totally legal if he's not used to advertise it, although the tricky part could be holding that the cameo comes from public domain work and not later, still-copyrighted derivatives.