On the RPi usb power cable

We have tested several USB cables. As many USB devices do not use much power, the resistance of the conductors in the cable is irrelevant. At 5 ohms and 50mA of current the voltage drop will be about 250mV. This is acceptable for most devices (and that's how it should be: the USB specifications require tolerance of such voltage drops).

On the other hand, some devices require more power. Those often have their own power supply so they too will work with the provided cables. And thirdly, phones that charge off USB will simply not charge as fast as they might with a good cable. In short, you won't often notice a higher resistance in USB cables.

However at 500mA a 5 ohm resistance will cause a 2.5V voltage drop. As the Model B will draw a peak current of 700mA, the voltage drop over such a cable will be quite unacceptable.

The cable that comes with a modern phone that charges using the micro USB port will probably be fine.

There is an excellent experimental analysis of RasPi USB power cables at element14. The author cuts open cables to show you how ridiculously thin the wires sometimes are, and shows how those affect 5V using oscilloscope traces.