I see more and more people that head on working with the I2C interface. That makes me glad, since we can interface many serial devices on just 2 pins with Arduino, instead of using several SoftwareSerial objects and many pins.
I'm not sure if you guys have seen John's I2C LCD and keypad project. I don't think he has made a shield, but that isn't that hard to make. Only one thing I would add to his code, that is regular serial interface, perhaps selectable with a jumper on board. Why? Because sometimes it's handy to plug it to see the text that is usually sent to the debug window on the PC.
Of course, using a mega168 simplifies the code, I like that, but adds cost to the shield for the same functionality.
Anyway, I'm looking forward to see this project done, I might buy a PCB to give it a try!
Edit: I found this new library, perhaps it's the way to go for compatibility mode...