We've started rolling out Android 4.1, Jelly Bean, to Nexus S phones on a number of carriers with more to come. Enjoy! android.com/whatsnew/
—
Google Nexus (@googlenexus) July 20, 2012
On Google Plus, Google elaborated a little more on the carrier situation:
We’ve started rolling out Android 4.1, Jelly Bean, to Nexus S phones on a number of carriers including T-Mobile, H3G, O2, Rogers as well as Vodafone in most countries, with more to come. Enjoy!
[Foot tapping.]

