Unlock bootloader using fastboot using Ubuntu/Linux
Linux users: If you're stuck in "waiting for device", make sure you run fastboot as root, such as:

$ sudo ./fastboot oem get_identifier_token  
...  
(bootloader)      
(bootloader) &lt; Please cut following message &gt;  
(bootloader) &lt;&lt;&lt;&lt; Identifier Token Start &gt;&gt;&gt;&gt;  
(bootloader) C8750A7847621A009AAC7949E9B41A52  
...  
(bootloader) 7206C18600834B50AB6FA9F4A9AFC71C  
(bootloader) &lt;&lt;&lt;&lt;&lt; Identifier Token End &gt;&gt;&gt;&gt;&gt;  
OKAY [  0.004s]  
finished. total time: 0.005s