Does there exist an (accepted) standard for I2C connectors and pinouts?
As mentioned in the ftpedia - many thanks for that - an Arduino can be connected to an RoboTX controller by a 6 pin (2*3) pin connector. Already tested, works fine 
This connector seems to be used with several readily available I/O modules. Unfortunately only 4 of the 6 pins are described, what about the other (middle) pins? Can they be used for interrupt signals or strobes?
DrDiettrich:
Does there exist an (accepted) standard for I2C connectors and pinouts?
Doesn't look like it.
The reason is that it is designed as its name suggests - as an inter-IC bus, primarily for use on a single PCB. It may be used for multiple modules in one instrument, but that is as far as the design intent goes. Using it for pluggable modules as with Arduino pieces, is not the intent..
DrDiettrich:
an Arduino can be connected to an RoboTX controller by a 6 pin (2*3) pin connector. Already tested, works fineThis connector seems to be used with several readily available I/O modules. Unfortunately only 4 of the 6 pins are described, what about the other (middle) pins? Can they be used for interrupt signals or strobes?
Well, that is the problem. It is somewhat proprietary to Fischertechnik, therefore not an accepted standard. If it was, you would have your answer already. 
The I2C specification includes an extension into 10 bit addresses, so it doesn't look to me like restricted to a single PCB. But it certainly is not easy to attach that many devices to a single bus, driven by uC output pins.
Conrad Electronic offers several modules with an 6 pin connector, and documents pin 3 as TxD and 4 as RxD, for use with their (proprietary?) C-Control I product family. But I don't understand how multiple modules would communicate over shared TxD and RxD lines. At least this spec doesn't look like I2C made. Perhaps the TxD and RxD are primarily used in programming the uC of the boards (if present).
Nope, no standard whatsoever. I wish people would agree to one for I2C/vcc/gnd on a 4-pin dupont connector, at least. I've been using the pinout of the BMP085 modules (Vin,GND,SCL,SDA) for my 4-pin connectors carrying I2C just so I have some level of consistency.
DrDiettrich:
Does there exist an (accepted) standard for I2C connectors and pinouts?As mentioned in the ftpedia - many thanks for that - an Arduino can be connected to an RoboTX controller by a 6 pin (2*3) pin connector. Already tested, works fine
This connector seems to be used with several readily available I/O modules. Unfortunately only 4 of the 6 pins are described, what about the other (middle) pins? Can they be used for interrupt signals or strobes?
Well get out your ohm meter and see where those two pins go! Do they go into the chip? Well look it up, what do those pins on the chip do?
If DrDeitrich's connectors are the same pinout, my first thought would be that the connector is power, ground, I2C SDA,SCL, and serial Tx and Rx - thus allowing the header to be used for a module with either serial or I2C communication.