Specifically, the charging unit that came with my HP Touchpad is a wall-plug unit with a female USB adapter on the other side (e.g. this pic on pcmag.com). Will this work for anything that charges with USB (my thought being the USB voltage/current rating is standard?) For example, I have a wireless headset that charges via USB on my computer, could I use the wall adapter for that without damaging the battery on the wireless headset?
HP TouchPad charger has output of 5.3V at 2.0A. My Nokia phone charger's output is 5.0V at 1.2A. and TouchPad complains of insufficient current for charging when connected to it or to a computer. At the same time, I've charged the phone off the TouchPad charger a few times without any issues so far although I was worried that the higher current could damage it.
As long the adapter has 5 volt output (which is the usb-standard) you could not damage your device.
However, some devices won't charge with those adapters.