Hi, I saw you make a reference to this topic in http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1287952386
I see your topic is quite old, the first thing I would definitely do is upgrade to Arduino 021, I think only after 020 there is official support for the Mega2560. I think the ISP software (are you using the one from examples?) is updated too.
Regarding the USB connect error, I can imagine that the nano is not able to power the mega, but as I am a rookie too, that's just a wild guess.
One other thing, I found out in the pins_arduino.h file that for the Mega not pins 11, 12 and 13 are to be used, but resp. 51, 50 and 52, you could try those, maybe your there is something wrong with your ICSP header on the Mega.
#if defined(__AVR_ATmega1280__) || defined(__AVR_ATmega2560__)
const static uint8_t SS = 53;
const static uint8_t MOSI = 51;
const static uint8_t MISO = 50;
const static uint8_t SCK = 52;
const static uint8_t SS = 10;
const static uint8_t MOSI = 11;
const static uint8_t MISO = 12;
const static uint8_t SCK = 13;
Hope this helps in any way.