It seems that the catchy tune that you hear playing at the end of Portal will be a downloadable song for Harmonix's mega hit rhythm game Rock Band, according to this blog post from a Harmonix Staff member. While attending Valve's GDC party, the song was played by a real band and then afterwards, played using Rock Band. The song that is innocently sung by the evil computer that terrorizes you throughout Portal will be coming, but when, is still unknown. This also helps to prove that the list of DLC Rock Band songs that was hacked on a recent downloadable song file may all very well be coming at one time or another, since this is the fourth song to be confirmed that has appeared on it.