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 powersupply so they too will work with the cheap 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 about 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.