Try a different cable. I ran into a data-only cable once. Also, does usb do the same thing as hdmi - device sensing for power, data, etc?
Also, you know that usb wall adapters are only about five buck s a pop, right? Why go through all the hassle of fabbing up a custom setup?
Sensing depends on the upstream device. Power consumption depends on both.
You can have "dumb" devices on both ends (USB coffee mug heater and wall wart from 7-11) or overly smart devices on both ends (apple stuff).
The reason apple stuff is weird is as Nick said. To claim USB compatibility, they must -accept- the USB charging specs (500mA), but then they tweak their stuff to have an additional high current capability (1,2A). So both devices then must check to see if the other end is special. Apple isn't the only company to do this ind you, others do as well.
Fortunately, its pretty easy to tell, there re really only a few options:
D+{float,5V,gnd,xxV) {shorted to/not shorted to} D-{float,5V,gnd,xxV}
The standard is the simplest--shorted floating pins, as its the easiest to signify "I am not a negotiating smart device, so send me whatever my input impedance allows at 5V but no more than 500mA (ie, maximum input impedance of 10 Ohms).
Since virtually everything smart uses buck/boost chare controllers, they can effectively "set" their apparent input impedance to whatever they want, and the chargers D+/- pin settings make it simple to do so, not to mention easier than monitoring the voltage of the +5V line.
Also, smart chargers will monitor their output voltage and go into a reset if the input impedance on the connected device is too low, rather than sag voltage below 4.9V and potentially fry their output regulator with excess current.
