i2c over gpio - level shifter
There are specialized ICs for I2C level translation.  PCA9306 (down to 1.8V),TCA9517 (down to 0.9V), FXMA2102 (down to 1.65V).  IC level shifter would be my choice.

@Laszlo had mentioned AN97055 by Philips (now NXP).
Excerpt from that app note (p.13).


  3. CHARACTERISTICS OF THE MOS-FET’S.
  
  The requirements for the most important characteristics of the
  MOS-FET’s, used as bi-directional level shifter in an
  I2C-bus system with max. 6V and min. 2.7V levels, are
  listed below. The values are intended as an indication and may be
  adapted for other supply voltages, other logic levels and/or other
  applications.  
  
  Type : N-channel enhancement mode MOS-FET.
  Gate threshold voltage : VGS(th) min. 0.1V max. 2V
  On resistance : RDS(on) max. 100 Ohm @ ID = 3mA, VGS = 2.5V
  Input capacitance : Ciss max. 100 pF @ VDS = 1V, VGS = 0V
  Switching times : ton toff max. 50 ns.
  Allowed drain current : ID 10 mA or higher.