UPDATE 3/22: Thanks for all the supportive comments! You will be happy to know that due to the incredible support (that I was not expecting) I am working on another tune and making it even better than this one!
I've seen a lot of questions about how this was done (and a few about it being fake - which I can assure you it's not) so here's the sort explanation. The laser beam makes sound when it strikes the plate. By turning the laser on and off hundreds of times per second it acts almost like a speaker cone moving back and forth hundreds of times per second - making various pitches. Some pitches are off key and will always be due to the laser "only" being accurate to .001". Due to how close different note frequencies are together it would need to be accurate to .0002" or better (totally useless for industrial marking) to always be on key. The rounding up/down to .001" gets the notes "close enough" but still off by a few cents.
UPDATE 3/21: I received word back from Valve and it looks like they already have enough people making Portal merchandise, so for the time being the plates are all mine. Miiiiiiiinnnneeeeee!
Still Alive by Jonathan Coulton played by an Automated Laser Corporation 20 watt fiber laser on a stainless steel plate.
It took quite a while to get it this far. Laser markers weren't really meant for making music and there are a few parts out of time but I'm sure GLaDOS would approve.