I've got an initial design for LCDuino. A board is up on OSHPark.
LCDuino is an LCD backpack. That is, it's the same size as a 2x16 LCD display circuit board, with mounting holes that line up, and there's a SIP header that lines up and connects to the LCD.
This backpack has an ATMega328 TQFP, an FTDI FT232RL, and an MCP23017 i2c GPIO chip. The FTDI chip is configured to connect to D0 and D1 just like an Arduino Uno. DTR is capacitor-coupled to the !RESET line so that the Arduino IDE can upload sketches with the right bootloader. A4 and A5 are configured as an i2c bus and fed into the MCP chip, which is in turn hooked up to the LCD, just like the AdaFruit RGB LCD shield.
The board has two DIP headers. One is intended to hook to a board with the buttons on it, and the other is intended to go to a ribbon cable leading to a breadboard. D0-13, A0-4, A6 & A7, the i2c bus (SDA & SCL), AREF, !RESET, +5 and GND are available.
The board will have a micro-USB connector on it which will both supply power and provide serial I/O for sketch upload and serial monitor. Initial bootloader installation and fusing will be done with a jig on the GPIO connector to adapt it to ISP.
Here's the full schematic: