I took the rock-paper-scissors comment to mean that each race has a chance to win, and a chance to lose. I've heard many claims that "Zerg can always beat Protoss, Protoss can always beat Terran, and Terran can always beat Zerg" or something like that, which would be a true rock-paper-scissors relationship, but for every "guaranteed" strategy for some race to beat one of the others, someone always manages to beat it anyway. That's the beauty of the game. There are so many variations to playing each of the races that none is "guaranteed" to always win, because you don't know what the other guys are doing.
The Zergling Rush is definitely a tried-and-true "strategy" that seems to work most of the time, provided you can find the enemy before he has a chance to build anything, but I've read elsewhere that Blizzard has taken steps to "improve balance" in the new game. Given that the balance was just about perfect to begin with, some people have taken this to mean that things like the Zergling Rush will be a thing of the past.