Seems like a lot of people are having these reboots/shutdowns while listening to music (streaming or local). It also doesn't seem to matter what app you're using to listen to music. I really hope this can be fixed through a software update...
Not sure, but it may not be the GNex. My Xoom running the official 4.0.3 OTA rebooted on me yesterday while listening to music with Power Amp. I wonder if there's something with the audio codecs that's causing this. Wireless states don't seem to matter. I've always been on 3G (with LTE disabled) when my Nexus rebooted, and my Xoom (WiFi only) had the WiFi radio turned off when it rebooted. Alywas been while listening to music of some kind...