[SOLVED]:
As suggested, I solved updating the firmware on the ATmega8U2.
Anyway, note that the instructions on the official site
are incomplete! In fact , in the paragraph
"Reset the 8u2" the picture showing how to reset the board concerns only Arduino Uno.
The right way to bridge the reset pins on Arduino Mega 2560 is correctly showed here:
http://www.wayneandlayne.com/blog/2011/02/16/fixing-linux-firmware-issues-on-arduino-mega-2560/
Note that if the pins are not reset in the proper way,
when you launch
sudo dfu-programmer at90usb82 erase
you get the following error:
dfu-programmer: no device present.
Thanks a lot, bye