retrolefty:
ScottAS:
Sorry guys, you are correct. For some reason I had it in my mind that power came in through UVCC on the 16U2, was regulated there and came out of it's VCC port to supply the 5V line. Seems silly, but this is the only thing that made sense to me, because the 16U2 is the only thing I saw USBVCC connected to. Now I see that it's connected to the MOSFET down below as well...Still not correct. You must be looking at a different schematic then I am? The 16U2 chip has it's Vcc on chip pin 4 and it clearly wires to the board's +5V bus, not to the USBVCC bus. The USBVCC voltage from the USB connector goes only to one place, the T1 FET switch.
http://arduino.cc/en/uploads/Main/arduino-mega2560_R3-schematic.pdf
The schematic you linked shows USBVCC connecting directly to UVCC on pin 31 of the Atmega16U2. Yes, its VCC is connected to the 5V bus.
EDIT: I'm still confused about this though. If only USBVCC was connected to UVCC, and VCC was disconnected from the 5V bus, would 5V still come out of the VCC pin?
EDIT2: Ok, I think I see on the datasheet that 3.3V will come out of VCC if it is not pulled up by another source. Still a bit muddy to me though.