It depends how it's wired. The way iDevices detect a mains charger is slightly different to the Touchpad. See the circuit diagram below.
With a standard cable you will get:
51k/(51k+75k) * 5v = 2.02v on Data +
51k/(51k+43k) * 5v = 2.71v on Data -
From the HP implementation above you can see that the TP is expecting the same value on both Data + and Data - namely:
300k/(250k+300k) *5v = 2.72v
If you use a cable with the data pins shorted together you effectively put the 43k and 75k resistors in parallel between +5v and data and also both 51k resistors in parallel between data and gnd. This will give you:
43k x 75k / (43k + 75k) = 27.3k effective resistance +5v to data
51k x 51k / (51k + 51k) = 25.5k effective resistance data to gnd.
Finally we can calculate the voltage on both data lines with this arrangement
25.5k/(27.3k + 25.5k) * 5v = 2.41v
You can see this is a bit lower than the TP is expecting but it might work for 2 reasons:
1. Resistors are notoriously vague in their value and can vary by +-5% quite easily.
2. The TP is likely to allow a reasonable range on the expected 2.72v
If you want an easy* way to get an iDevice charger to work with the TP, short the data pins at the TP end and cut data + on the charger end. This will make the Data - voltage (2.71v) appear on both Data pins which is what the TP wants from its own charger)
*easy is a relative term, good luck managing to short the pins on something as small as a micro USB plug
As ever, YMMV. I am not responsible for any damage you may cause.
PS. I'd like to say thanks to Wikitronic for his amazing circuit diagram tool at
http://webtronics.googlecode.com Without the diagrams this thread would be much harder to follow.