The voltages on USB Data+ and Data– pins tell the iDevice how much current the USB port can supply. Let me sum up my research and all the available information on this topic.
[UPDATE: 2013]

USB pinout

Pin Wire color Description
1 Red +5V
2 White Data–
3 Green Data+
4 Black Ground

Apple charger specs

charger voltage max. current max. power
USB 2.0 5V 500mA 2.5 Watt
iPhone charger
5V 1A 5 Watt
iPad charger (A1357) 5.1V 2A 10 Watt
iPad 4 charger (A1401) 5.1V 2.4A 12 Watt

Charging modes

charging mode Data– Data+ actual current
drawn by my iPhone4
actual current
drawn by my original iPad
actual current
drawn by my iPad3
500mA mode 2V 2V 100mA before handshake,
240mA, 350mA, 440mA on my desktop on different USBs (wtf?),
550mA when connected to Macbook Air (incl. running from battery)
100mA before handshake, 480mA desktop PC,
920mA Macbook Air USB
100mA before handshake, 480mA desktop PC,
920mA Macbook Air USB
1A mode 2.75V 2V 520mA 800-900mA 950mA
2A mode 2V 2.75V 550mA 800-900mA 990mA
2.4A mode 2.66V 2.66V ? ? ?

The current drawn by a device remains constant even you unlock it and start some power hungry app.
You can notice that the currents seem a bit strange, but I’m looking into it. Maybe the devices need to be discharched completely – I’ll post an update later [april 2013]. EDIT: A completely discharched iPhone draws the same power.

Resistor configurations you can use in your project

Required Data+/– voltages can be set with a simple voltage divider circuit. You can select different resistors based on what you may have in stock. Data+ and Data– voltages probably don’t need to be precise but approximate. There’s some examples I found on the internet which should work:

resistor 500mA mode 1A mode 1A mode
(genuine iPhone charger)
1A mode 2A mode 2A mode
(genuine iPad charger)
2.4A mode (could work)
R1 75k 33k 75k 75k 1% 33k 43k2 1% ? 51k
R2 50k 22k 51k 49k9 1% 22k 49k9 1% ? 49k9
R3 75k 33k 39k 43k2 1% 33k 75k 1% ? 51k
R4 50k 47k 51k 49k9 1% 47k 49k9 1% ? 49k9
tested by me ok ok ok ok
device              500mA mode 1A mode 1A mode 1A mode 2A mode 2A mode 2.4A mode
iPhone 3GS ? 240mA? ? ? ? ? ?
iPhone 4 ? 380mA ? ? 620mA ? ?
iPad 3 ? 620mA ? ? 1.1A ? ?

Detailed table for 1A mode (33/22/33/47k set):

voltage              iPhone 4 iPad 3
≤ 4.3V 0mA
nothing happens
nothing happens
4.35V 0mA
nothing happens
nothing happens
when voltage slowly drops from higher voltage, ‚Charging is not supported with this accessory.‘ alert screen pops up and 180mA is drawn
4.5V 180mA 280mA
‚not charging‘ in status bar
4.66V 180mA
‚not charging‘ screen
‚not charging‘ in status bar
4.7V 230mA 380mA
‚not charging‘ in status bar
4.85V ? 480mA
‚not charging‘ in status bar
5V 380mA 620mA
‚not charging‘ in status bar
5.2V 420mA 660mA
‚not charging‘ in status bar
≥ 5.25V ? 0.7A
‚not charging‘ in status bar

I’ll try the remaining configs later this month (april 2013). Stay tuned!

Voltage divider circuit

Useful links

Connect and charge iPhone/iPad with your old Kenwood headunit
ricpaul’s posts on forum
timothyb’s DIY iPad 2 USB Charger