Actually, this has everything to do with portal. After the Pack was released, every game in the pack had an update, adding potato references, or references to other games in the pack, or portal. For example in the AAAAaaaaAAAAAAARRRRRGGG game where you fall and stuff, there are these "news flashes" at the end of a level, which I think were added like a week or so ago, and some new ones are still being added I think. A couple of days ago, one of em said "I can't stop thinking about these poor lab rats." The next day, valve made a comic like the ones for tf2, and it's called Lab Rats. That's just one example but there are a lot more.
http://valvearg.com/wiki/Valve_ARG_Wiki
Go there to read all the stuff about it, it's awesome.