I have a Galaxy Nexus 'maguro' and a laptop with Ubuntu 12.10. I'm trying to install Ubuntu Touch preview. I installed the adb and fastboot packages, and adb detects the device when it is booted in to Android. But when I do a reboot-bootloader, it no longer turns up in the devices list.
"Boot loader mode" is also known as "fast boot mode" hence you need to use fastboot utility to access the device. This utility is part of Android SDK, and can be found in the same location as adb -