At what point does crosstalk on I2C become a problem?
Crosstalk is usually not a problem, when the I2C bus is contained within a single PCB and pull-up resistors are properly sized.  As the size of the bus grows, bus capacitance will become a problem before crosstalk becomes a problem. 

If you think that capacitance between SDA and SCl is high, run a grounded trace between them.



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