I've done master and slave in software on PIC.  I think the master runs
slower than most hardware driven masters, but who is counting cycles when
you can do it all on 2 wires.  The slave runs at a compatible speed to the
master software.  Our most complex I2C setup is:

  Master        PIC with lots of logic, LCD, keypad, etc
  0             Clock/alarm (via interrupt line) device for time keeping, 24 hou
r
wakeup alarms, etc.
  2             8 kByte memory device
  3             PIC with 6 line serial link to a PC.  Non-I2C interrupt line to
master
when a message arrives

The interrupt lines are needed to trigger the master to read from the
device when an event occurs.
  <edtodd@sni.net>    Ed Todd