Davide Gironi has posted an I2C multiple HD44780 AVR Atmega library:
This library implements a driver for HD44780 lcd connected through PCF8574 port expander.
Data is transmitted using only 2 wire over i2c with the PCF8574.
This library can drive up to 8 LCD concurrently.
Lcd driver is based upon Peter Fleury’s lcd driver
HD44780 to i2c library its based upon this library
An Atmega library for multiple HD44780 LCDs – [Link]