Is there any definitive I2C pin-out guidance out there? Not looking for a "STANDARD"
I've recently rolled my own as far as I2C connectors go. The connector itself is not very important, right now I'm just using 100mil pitch header (usually female on board so it's not so pokey when not connected), but any 4plus pin connector will do. Additionally, I'm using the P82B715 from TI as an I2C bus extender. This overcomes the capacitance issues associated with running long I2C drops off board, which as people have been saying, I2C was not intended for initially. I did try many different combinations, like in the examples you gave and I noticed absolutely no difference in performance. I believe this is because I2C is relatively slow, interference between SDA and SCL is not much of an issue. Basically the rise time for voltages (when interference will occur) on the bus are much much smaller than a bit length. So, that may not be what you want to hear, but it does afford more options. Personally I went with [VCC, SDA, GND, SCL] to be easily routed to/from this chip and also be immune to a VCC/GND mix up when plugged in backward.
When it was first established, the I2C (Inter-Integrated Circuit) bus was only intended to connect chips on a single PCB assembly. It was never intended to be used on cables to connect multiple boards together, and therefore, no connectors for that purpose were defined.
The only "standard" I2C-based external interfaces I'm aware of are the short-lived ACCESS.bus for connecting user interface devices to computers and the VESA Display Data Channel used to retrieve monitor information over VGA, DVI and HDMI connectors.
No standard pin-out, no standard connector. The I2C standard isn't real amenable to this sort of thing. It's specified at the bus level, not the device level. For example, when you'd like to plug in an I2C device, do you know by the standard whether the pullups are on the host or on the device?? No. Plenty of other stuff you don't know, either, like where your cable capacitances are, what Vcc needs to be ....
In fact, there are even I2C devices that need extra lines for interrupts and other plain old digital I/O. How do you add those to the standard, if you can't get agreement on how many pins you need.
Bottom line, if your looking for portability and stability in your interconnects, I2C and SPI aren't where you need to look.