Okay, I've been doing some tinkering.
Here's a schematic & layout for a DIP based Duemilanove style 366/644/1284 board.
Its intended to have the 1284 mounted on socket strips, so there are a couple of caps underneath the part.http://www.dipmicro.com/store/HDR40X1FM
I'll do a version with a surface mount part also
If there is interest, I'll spring for a batch of 10 or 20 PCBs, and folks can obtain their owm material and build them as they wish.
The IO pins are as defined for my Bobuino, I'll post the files for that too, designed to be fully shield compatible.
The extra IO of the 1284 series are brought out to another connector. The Analog header has all 8 Analog inputs as well.
USB power or onboard regulator power. Regulator may be surface mount, or thru hole part may be used.
Reset - jumper for reset from FTDI, or leave it open for no reset.
I2C: Jumper to have A4/A5 go to shield, or have SCL/SDA go to shield. SCL/SDA also available on the extra IO header.
There is an onboard 3.3V regulator as well, don't have to rely on the 50mA from the FTDI chip.
Board is a little longer then an UNO, due to the bigger chip.