OK, got it! Looking at the new USB-IF battery charging spec. 1.0 it looks like this.
Wall chargers should have the D+ & D- pins shorted together & floating.
PCs & some other devices should have the D+ & D- pins connected to gnd. through 15k resistors.
Well I tested the musicPower Dual Port charger from Walmart (works with Zune) & it shows about 15k to gnd. on the data lines, so apparently the Zune wants to be hooked up to a PC no matter what & can't detect a properly configured wall charger. There is something else in the circuit on that charger 'cause I seem to be charging a cap while measuring from the data pins to gnd.
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