Following http://arduino.cc/en/Tutorial/ArduinoISP didn't work. I connected pins 50 to 53 of both mega2560s together (as per http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1279287827 this recommendation) and 5V and GND together.
When I select Burn Bootloader -> w/ Arduino as ISP the output is:
Burning bootloader to i/o board...
Error while burning bootloader.
avrdude: stk500_getsync(): not in sync: resp=0x00
The RX light on the ISP arduino flashes twice right after I select Burn Bootloader -> w/ Arduino as ISP; no other lights flash. The L light on the dead mega is still lit constantly when the live mega is plugged into USB; when the power is cycled, the two megas flash their L lights identically, and then the good mega shuts off the L light, the bad mega leaves it on. Both power leds (ON) are lit.
Some similar ArduinoISP with atmega2650 posts:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1287952386 this post mentions there could be some updates to ArduinoISP sketch necessary to program a mega, due to the increased address space of a mega; its been added as a googlecode issue.
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1279287827 this person was having trouble using ArduinoISP on a mega as well... though perhaps it was a power problem.
I'd like to check the voltages on the 8u2 chip, but since it's surface-mount that seems very difficult...
Out of ideas for the moment, other than maybe getting an ISP programmer, maybe this one USBtinyISP AVR Programmer Kit (USB SpokePOV Dongle) [v2.0] : ID 46 : $22.00 : Adafruit Industries, Unique & fun DIY electronics and kits to try with avrdude. There's a few people working on using ISP direct to the 8u2 (Arduino Forum )