If you want to try a signed copy of the official Android 4.1 Jelly Bean build, then now is your chance. It has just hit the Internet and seems to only be working on the 'takju' (GSM) GALAXY Nexus' given out at Google I/O 2012, reports Android Police.
You can download it for yourself here, coming in at 156MB. The dev community should smash into this pretty quickly and GALAXY Nexus owners should be munching on those delicious jelly beans shortly.
Please do let us know if you get it working, we'd love to hear from anyone running Jelly Bean.