I've noticed that the rebus seems to have gone the way of the dodo for the most part, having been replaced with the emoticon. :(
I guess part of it is that many symbols that we use no longer have common names.
I was helping another coder here debug code earlier, and I told him to insert a "bang" at the beginning of a clause. He looked at me quizically and said, "Bang? What's that?" He's had the same reaction to me calling an asterisk a "splat."
Most people get ones like:
I ♠ your cat.
I can see some causing issues, like:
I ♣ baby seals.
...doesn't mean the same if you read it as "I shamrock baby seals."
Others depend on your pronunciation in order to get them, like:
I ♦ your arms tonight.
(Hint: Think 80's popular rock.)
What other types of jokes or means of communication that were popular in your youth have gone the way of the telegraph?