Two hard nuts to crack, or three.

Triple badlander - for those who can't count. Google's prime number billboard versus BMG's trade poster.

Many people recall Google's prime number billboard of 2004, google even posted about it in their own blog.
The poster depicted here hung in downtown Seattle in fall 2004.
This math problem on a billboard was google's ingenious recruiting campaign - and positively everyone thought it was a brand new idea.
 

But we know better than that, don't we? ;)

Ground Zero did the same thing in the E3 Trade show program (Electronic Entertainment Expo) to attract top young game developers, back in 1998. In C++  they wrote "Does your publisher understand you?" and all those who could read it stopped by the booth for BMG, the sender.

Art Director: Brian Tortora Writer: Johhny Agnew  

Can't forget the EA Billboard in Canada that spelled out "now hiring" either.

Now currently I've seen ads on the same premise, with the concept that it's written in code and only the target market can read it.

​However, I forgot to note down where I saw this so if anyone out there can help refresh my memory... (images can be posted in the comments!).