All tested boards hang right out of the box with specific power ON sequence

Using Arduino IDE 1.x
2

hi,
a couple of things to try:

  1. cut the RESET-EN link. this will prevent a crash of the 8U2 from affecting the 328p.

  2. place a diode as suggested in this thread:
    http://arduino.cc/forum/index.php/topic,64256.0.html