Err... I really don't mean to hijack the thread, but I thought if I created another one I might be clogging up the site... :/ What about fictional characters? In a WiP of mine, my MC has a conversation with many fictional characters. I could work my way around it, but it would have that extra great touch if I could actually mention Spiderman, Dorian Gray, the Simpsons, etc... characters people in the future would hopefully know. I would not say anything libellous of course, but obviously a bit of their personality would come across. I understand for Dorian Gray it's a bit different because he's nineteenth century, but what about relatively modern superheroes and cartoon characters??
i'd say it's probably ok to just mention them in passing, but since they're trademark protected, you may well need the owner's permission, if you want to include them as minor characters... i'm sure steerpike will have more reliable info on the matter... counselor?