You need to power down the device after done flashing the ICS 215 kernel, or do the fastboot reboot command. My phone wasn't real keen on fully rebooting, either. Vol-Dn + Power didn't seem to take well, and the phone would instantly pop back to fastboot. It took a few tries before actually registered right. I considered pulling the battery at this step to force it to power down, but it caught and worked before I got to it.When I reboot the device it just dumps me back into fastboot mode.