The quote "You are what you do" appears to originate from Jean-Paul Sartre, a 20th-century French philosopher. This particular sign can be found obscuring the view from my office, which lies somewhere between "the" and "exceptional". :)
I understand that the view held by Sartre was that you are responsible for who you are, and what you do dictates the person you are - which is a fair statement, I think. So, I guess that the message today isn't as much about Cape Town as it is about deciding to be awesome.
So, go ahead, do the right thing, be exceptional! ;)