I've been testing the Galaxy Nexus with JB 4.1.1 and this is what happened yesterday. Has anyone seen this screen?
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That screen means your device is in "download" mode, which allows it to be reflashed using Samsung's Odin utility (or the open source alternative Heimdall). It's accessed by holding the volume down button when powering on your Galaxy Nexus.
Also, to clear up some confusion, this is not Fastboot mode - that is different. You access fastboot mode by holding down both volume up and volume down while powering on your device, and it also gives you access to a bootloader menu which you can scroll through using the volume rocker. The image on the screen is also different, and your device will not respond to fastboot commands in Odin mode. This is what the fastboot screen looks like - as I noted, you can see that the most obvious difference is that the background image is not the same: