While we were playing Portal 2 the other night we studied the graffiti in the first Ratman's Den. It essentially depicts Schrodinger's cat paradox, which is if you lock a cat in a box with a flask containing a poison and a radioactive source. When a Geiger counter detects radiation the glass shatters and the poison is released. However, the box is sealed and at no point would an outside observer be able to tell if the cat is alive or dead without opening the box.
To connect this point the events of Portal 2 happen after the events in Half-Life 2 wherein an epic struggle is unfolding on the Earth's surface. In the Aperture facilities the epic struggle Chell has with battling one A.I. and then another seems trivial in comparison. Regardless of whether Chell fails, the facility blows up, or she saves it and everyone is okay, no one would know unless they took a peek inside.
This means one can argue that Valve purposefully put this reference in here as an allegory for Portal 2, or rather Portal 2 is an allegory for the Schrodinger's cat paradox. This can be a fun idea for someone to play with, as Portal 2 is riddled with paradoxes, it's mentioned the A.I.s within can't resist paradoxes, and paradoxisms is a legitimate literary motif. Just some thoughts for someone(s) to explore if they wish.