If you hold down the power button for 20 seconds (or Reason: 10 is shown on the console) the system will boot into tech support mode, dropping you in a shell within the initial initrd. In this mode the system has a root shell on the console and also listens for telnet connections.
Note that starting with firmware 4.1.7 it automatically opens a tunnel connection back to Netgear in this mode, giving them easy access to your box even if behind a firewall.
The console driver is somewhat limited (fails if you send data too fast) so it's recommended to use telnet.
The telnet login is "root" with password "infr8ntdebug".
In this mode the initrd image is complete except for the /linuxrc boot file. This can however be recovered in /proc/1/fd/ if you are interested in what the system is doing on bootup.