Soft i2c with Sensirion SHT21 sensors on Arduino Mega
Just for completeness, I believe Jonny Flowers has found the solution himself: http://arduino.cc/forum/index.php?topic=99832.0

He's also put his code online: https://github.com/mrjonny2/PortsSHT21

His solution is to edit the Ports.h file's

uint8_t portNum;

inline uint8_t digiPin() const
    { return portNum ? portNum + 3 : 18; }
inline uint8_t digiPin2() const
    { return portNum ? portNum + 13 : 19; }
inline uint8_t anaPin() const
    { return portNum - 1; }


to

uint8_t portNum;

inline uint8_t digiPin() const
    { return portNum ? portNum + 3 : 18; }
inline uint8_t digiPin2() const
    { return portNum ? portNum + 53 : 19; }
inline uint8_t anaPin() const
    { return portNum + 53; }