Google will officially release the Android 4.1 aka Jelly Bean update to Galaxy Nexus users in mid-July but if you can't wait a fortnight, you can grab the OTA download directly from Google's server. The OTA link is the preview build of the Android 4.1 that was given to the developers at the Google IO yesterday.
Announced yesterday, Amdroid 4.1 is said to be one of the smoothest and fastest Android OS, thanks to features like Project Butter, improved camera interface, keyboard, Android Beam, Voice Search, offline voice dictation, Google Now etc.