Serial communication problem with Mega 2560

Right. Something has happened to my Arduino board (got it in march-april this year), which makes me unable to communicate with it via USB. I can't upload sketches or update the firmware.

I've tried the all the regular stuff that's recommended in the forum to get it working. The TX/RX leds never light up, not even when booting, but the 'L' led is always dimly lit.

I've tried it with Windows, OS X and Ubuntu, so it's not a OS specific problem.

This is what I get when I try to upload a sketch:

/usr/share/arduino/hardware/tools/avrdude -C/usr/share/arduino/hardware/tools/avrdude.conf -v -v -v -v -patmega2560 -cstk500v2 -P/dev/ttyACM0 -b115200 -D -Uflash:w:/tmp/build2222393668295502507.tmp/Blink.cpp.hex:i 

avrdude: Version 5.10, compiled on Jun 29 2010 at 03:44:14
         Copyright (c) 2000-2005 Brian Dean,
         Copyright (c) 2007-2009 Joerg Wunsch

         System wide configuration file is "/usr/share/arduino/hardware/tools/avrdude.conf"
         User configuration file is "/home/chris/.avrduderc"
         User configuration file does not exist or is not a regular file, skipping

         Using Port                    : /dev/ttyACM0
         Using Programmer              : stk500v2
         Overriding Baud Rate          : 115200
avrdude: Send: . [1b] . [01] . [00] . [01] . [0e] . [01] . [14] 
avrdude: ser_recv(): programmer is not responding
avrdude: stk500_2_ReceiveMessage(): timeout

If I try to re-flash the board with dfu-programmer I get the following:

dfu-programmer at90usb82 erase
dfu-programmer: no device present.

Strange thing is that the board is recognized in Windows as Arduino Mega 2560, and in Linux the dmesg output is (in OS X I get a popup telling me a new network interface has been connected or something like that):

[64347.976064] usb 2-2: new full speed USB device using uhci_hcd and address 24
[64348.178966] cdc_acm 2-2:1.0: ttyACM0: USB ACM device


I only have a pretty crappy iPhone camera, but the label on the chip below the USB port is: ATMEL MEGA8U2 1013 TH 9F4286-2