I don't understand why people are so up in arms over this one.
Playstation Magazine is NOT a Sony Publication - it is the same people as Electronic Gaming Monthly which is owned by Ziff Davis.
Advertising spots are for sale, and anyone can buy them....it's also possible that Gears of War bought a package that included the back page ad on multiple magazines and that's how it ended up there, though i'm pretty much guaranteeing that they specifically wanted it there - and probably paid a pretty good price for it. I mean, what better way to market it? You got people that are looking at PS Magazine, obviously owning at least a Playstation..but most likely a 2/probably a 3. Then on the back they see this ad for a game that is incredible, that they can't play....so now they've bought an advertisement for the 360 world as a whole, not just Gears of War - though it IS the selling point.
I'm sure no one was fired, if anything, I bet the marketing person got a bonus for such a big sale....I'm sure in most cases Sony has first dibs on ads in the magazine, but that doesnt mean others can't advertise in it for top dollar.