Social gaming juggernaut Zynga is poised to turn all those virtual cows and chickens into billions of real dollars through an initial public offering, sources tell All Things Digital. The company—which says its has around 250 million people a month playing Facebook games like FarmVille, CityVille, and Mafia Wars—was valued at $10 billion in its last round of funding, but is expected to price itself higher for the IPO. (More stock offering stories.)