Really? I thought the keynote explicitly said the SDK would be available today, but that neither the source code nor the binary updates would be pushed to the public (as opposed to hardware manufacturers, who now have ahead-of-time PDK access) until mid-July.
(edit: In addition, the Nexus 7, running Jelly Bean, while available to purchase today, will itself not be shipping until mid-July. I really don't think it is available yet.)
I cloned the manifest project and checked the tags; there aren't any 4.1+ tags present yet. I'd imagine there would be a JBQ post (like this, https://groups.google.com/group/android-building/msg/c0e01b4...) announcing Android 4.1 availability if the source were already available.
(edit: In addition, the Nexus 7, running Jelly Bean, while available to purchase today, will itself not be shipping until mid-July. I really don't think it is available yet.)