Just tried to hack up a USB power supply from a 5 volt, 2 amp switched mode wall slug. With USB D+ and D- floating it didn't work on two different tablets (HP and RIM). Then I followed Dave's power supply schematic which puts 2 volts on both with separate resistor dividers. That did not work either. The appropraite reference to look up is "USB dedicated charger". Seems that the USB folks standardized shorting D+ and D- together circa 2010. When I did that, it worked.