Here is an app note from Microchip describing how to setup, and implement slave I2C communication on Pic devices with MSSP modules. It also features assembler code for PIC18s and PIC16s.
Many devices in the PICmicro family have a Synchronous Serial Port (SSP) or Master Synchronous Serial Port (MSSP). These peripherals can be used to implement the SPI TM or I2C communication protocols. The purpose of this application note is to provide the reader with a better understanding of the I2C protocol and to show how PICmicro devices with the SSP or MSSP modules are used as a Slave device on an I 2 C bus.