Well for the backpack I've calculated approx costs.
The PCB is a tiny 1.5 inches by 0.75 inches which at BatchPCB means $5 per board.
It would be $2.50 if I can shave off a fraction off a inch.
At Futurlec for buying 100 of them its just over a dollar a board.
Components wise I can get them singly for about $3.50.
Buying in bulk would slash that amount.
Specs:
ATmega8 running at 16mhz.
Contrast control via trimpot.
Ability to turn the LCD's backlight on/off (power saving or for getting attention).
I2C, Serial and SPI all avaliable.
ICSP header for programming.
Software:
The software will be GPLed naturally. 
I'll probably put the Arduino bootloader on it so it can be easily updated.
The serial protocol will probably be compatible with other serial backpacks.
The SPI and I2C protocols will be a efficient protocol which I'll work out.
Sample code will be provided for interfacing it with the Arduino using all three methods.
So is there enough interest for me to make a batch of about 25 boards?