I just noticed this today and the last sketch I was running was an IR sensor, which was working fine. When I connect it through an external power source it works, but not through usb. Anyone know what could be happening?
Well the USB power goes through a 500ma thermofuse before it powers the rest of the board. Perhaps you have some external component or circuit that is drawing too much current?
Perhaps you can try disconnect everthing wired to the board and load the blink program and see if the on-board pin13 LED works.