My Yun was dead

got my Yun,
by accident I save unwanted config and viola went bricked.

got it back by
doing this ( from Arduino Playground - Yun )

Upload the YunSerialTerminal example via usb
Reset the AR9331 (white button near the leds)
Wait until you read "Press the [f] key and hit [enter] to enter failsafe mode": do that, press f and hit enter
Type cd /usr/bin
Type ./reset-to-factory-anyway

few details left out in the documentation.

open your arduino set the Yun board and serial port
set serial monitor to 115200 baudrate
and to Both NL & CR from dropdown

hit the Yun RST button

when ask type "f " in the text box next to "Send " Botton
if you see the linux ascii art you are in the linux side of arduino
then do
and last

in my case I had to reboot my computer ( maybe dns caching issues )

Hope that helps