Quick answer: by 2012. From Wikipedia: US regulations for ESC for all passenger vehicles under 10,000 pounds (4536 kg), phasing in the regulation starting with 55% of 2009 models, 75% of 2010 models, 95% of 2011 models, and 100% of 2012 models.
I would have bought ESC if it were available, any extra margin of safety would be greatly appreciated. Unlike ABS, accident prevention data for ESC has been proven.