On an Apple 12W USB Charger, how are the D+ and D- lines configured?
The D+ and D- (I think you made a typo in your question) lines must be held to 2.7 V to support 12 W charging.

Unfortunately I don't have access to the Apple MFi Specification (which would be the best source), but this forum entry and subsequent answers by TI employees indicate that 2.7 V is the correct value.

Furthermore, this schematic below uses a resistor divider of 43.2k and 49.9k on the D+ and D- lines, which supports the TI forum responses of 2.7 V.



This is the schematic of the ACT4523A 5V/2.4A iPad Car Charger (Page 13 of this document, for your reference)