Although they didn't specify when, Samsung just made an official statement confirming that the Nexus S would be upgraded to Jelly Bean. More than likely it will happen sometime this month, as Google already hinted at this probability. Here's their official statement,

Samsung will soon announce which additional devices are eligible for the Jelly Bean update. As the world’s largest smartphone manufacturer, Samsung leads the Android community with best-in-class devices like the Galaxy S III, and is creating new device categories with products like the Galaxy Note. Samsung has delivered the most Nexus-branded lead OS devices and we are pleased that Google will be bringing Samsung Galaxy Nexus and Nexus S customers the first experiences of Jelly Bean on a handheld device.
The Galaxy Nexus will likely get the update first, but it's good to see that the Nexus S should get it soon too.

Source: PhoneArena