Jelly Bean is coming to the Galaxy Nexus, as Android version 4.1
Android Police has some interesting bit of info in this article. While the whole article is about the Nexus tablet, the part at the end, written by the Android Police author, is interesting:
Update by Artem: I dug into our server logs and found multiple instances of Android 4.1 and plenty of references to Galaxy Nexuses running builds like JRN54 and JRN54F, adding credibility to the information above. Only a few weeks ago, a JRN build was listed as JellyBean with no reference to the OS version, which happens before Google assigns one officially. It looks like now they have. Sure, this information in our server logs could have also been faked, but based on my experience, it's so consistent that it's very likely to be legitimate.
In a nutshell, this means that it's highly likely Google is dogfooding Android 4.1 on Galaxy Nexus devices, since there are MULTIPLE references of it in Android Police' server logs.
ICS builds had build numbers starting with I, like ICL53F, IMM76D, etc. It isn't surprising that Jelly Bean builds would start with J.
When will it come? No one knows, but I'm pretty sure a release is imminent. It might not bring any major changes, it would probably be just another minor update like Android 3.0 -> 3.1, but since the code name is changing, it could mean quite many new features are on the way.