A few years ago I bought a supply of 30 pin cables so I'd have them on hand for when I worked on old tech like the ipods and iphone 4s I have hanging around, so I've still got several lying around.
I pulled one out and checked the pinout between the 30 pin end and the USB end. Here's what I came up with.
Looking at the USB cable on end, from left to right the pins are:
* GND
* Data +
* Data -
* +5V
These map to the 30 pin connector as follows (numbered pins 1 - 30 from left to right)
* Pin 15 GND
* Pin 16 GND
* Pin 23 +5V
* Pin 25 Data -
* Pin 27 Data +
Note that on the 30 pin connector, only those pins had an actual wire, with the exception of pin 1. However, no matter how much probing I did, I couldn't find any continuity between pin 1 and anything else. The shield in the USB is also a no connect; it doesn't connect with the shield on the 30 pin end, or anywhere else for that matter.
I found a pinout for the 30 pin connector that matches up with my findings.
[link|https://pinoutguide.com/CellularPhones-A-N/iPhone_connector_pinout.shtml|Apple iPhone (1G, 3G, 3GS, 4, 4S) Dock cable connector pinout diagram @ pinoutguide.com|new_window=true]
Hope something here helps you out.