I made a couple of these without the internal diode to drop voltage. My boss at work was given one and left it plugged in all day every day until it burned out. USB port voltage is too much. The cheap high amp diodes at Radio Shack also drop almost a volt, which reduces the voltage to what it would be if it were run on batteries, as intended.
The local auto parts stores sell these lights for about $10. The usb cable came from discarded ups backup data cables (proprietary usb to ethernet). The diode is a 6 amp 50 peak inverse voltage diode from RS.
The pictures below show it plugged into my mac laptop usb port. The room light was off for the first two pics.
With my finger over the flash