Before I explain, promise me to never to refer to GLaDOS as a he.
So basically you are a girl named Chell. The AI, Genetic Lifeform and Disc Operating System, or GLaDOS is using you as a test subject. But in reality she is just testing the portal gun.
Before I continue: back story link here: http://half-life.wikia.com/wiki/Aperture_Science
Ok. During Text Chamber 19, you get on a platform that caries you into a fire. This is because you are of no use anymore, and GLaDOS is done with her tests for now. You survive and GLaDOS mentions a Party Escort, which is a robot meant to deal with unruly subjects. You go through the orifices of Aperture Laboratories and find and destroy GLaDOS. The explosion caries out outside the labs.
Valve recently released a patch with an alternate ending. After you explode, the Assistant carries you back in, which connects Portal and Portal 2.
The song is foreshadowing that GLaDOS was backed up somewhere, and is Still Alive. :D
I didn't even know there was a beta, but i saw E3, and it seemed easy. Just like LBP made it seem easy... So I have no definitive answer for you, but I believe it will be around as long as it takes to make a decent level in LBP.