For those looking for a way and library to convert a parallel HD44780 compatible LCD to serial (I2C):
I needed the exact same thing and wired it up using a PCF8574 and modified the standard LiquidCrystal to talk I2C to the display.
The new library named “LiquidCrystal_I2C” can be found on the playground.
It should be function compatible (forgive me if I missed one) with both the standard LiquidCrystal library and the API recommendations listed by Dale.
With a few additional components it also supports backlight switching.
Connection details, PCF8574 datasheet and examples are in the zip filehttp://www.arduino.cc/playground/Code/LCDAPI