I've just today received an AdaFruit PID 501 5V 1A USB power supply, as suggested on the RaspberryPi products list, and a USB A/MicroB cable ordered at the same time on the PID 501's page. I'm seeing voltages on (both) my RaspberryPi model B's tp1 and tp2, presumably the source of the dreaded trouble initializing the SD card error I'm seeing.
Note: I didn't get the RPi(s) from AdaFruit, just the power supply and cable. (I ordered my RPis before I knew about AdaFruit.)
With nothing connected but the power supply and SD card, I'm seeing about 4.74 V on the test points. With HDMI connected, I see more like 4.69 V and even less with my keyboard and mouse plugged into USB as well. Additionally, if these voltages glitch lower with current spikes (such as when trying to write to the SD), my DVM is unlikely to catch it. (I see the same voltages on both of my model Bs.)
A further note: The A/MicroB cable listed on my invoice is PID 592, while what was shipped is labeled PID 4867.
I purchased these parts with the thought that they were suitable for running a RPi, including model B. If they should be, then I can think of several possibilities:
1. The PID 501 isn't keeping up its advertised 5.25 V under the load of a model B Pi, especially when running HDMI.
2. The PID 501 is making it, but the cable is too resistive (inadequate gauge of conductors).
3. The PID 501 and PID 592 are OK (presumably you've tested this combo and your customers are successful using it), but the shipped PID 4867 is too resistive. (If this is the problem, you may want to be careful what you are shipping.)
4. The AdaFruit parts are fine, but somehow my model Bs (ordered from Allied last July and finally received in December) require far more than the usual current (I consider this unlikely, since the ultimate source of all the Pis is presumably the same).
So I'm looking for suggestions on further checking before I as for an RMA. I'm an electrical engineer by training, so I'm willing attempt non-destructive disassembly of the PID 501, or destructive disassembly of the A end of the cable, to measure the voltage at the PID 501 output to further isolate the problem (I don't have a USB A to A M/F breakout connector), but not at the cost of not being able to return out of spec parts for refund or replacement.