I have done a lot of work on it, in fact it is a totally new design. I've simplified it because I think the previous version was too complicated for me to get running before the end of the decade.
Physically it's nothing like the Due, I wanted to go smaller (it's 2.2" sq), provide for debugging support, addressable "shields", external memory, control of a power (5A up to 30V) for an external device, system power shutdown/wakeup etc and I'm sure the Due will be dumbed down so I decided to go my own way. For better or worse
Apart from the fact that I won't be able to use shields I don't think it matters, after all there are many Arduinos that don't (Teensy, Pro mini etc).
What I hope to have is the same processor and therefore all the libraries and tool chain. When they first released details they said the Due would use the SAM3U4E, they have since said that they may be using a different processor but if you analyse the SAM3 range the 4E just seems the most appropriate, for example it's the only one with 5 serial ports and USB. So I think that was a smoke screen to throw off the clone makers.
Working on that assumption I decided to finish the design with a SAM3U4E, brief feature list as follows.
• SAM3U4E 96MHz ARM Cortex-M3 processor.
• A 102-pin backplane with up to 8 addressable daughter boards and provision for memory expansion and/or fast IO.
• Debugging support with JTAG, two dedicated debug pins and an on-board 7-segment LED.
• Software control of power to the entire system and various subsystems.
• Provision for up to 32Mb of external SRAM.
• Battery backup for external SRAM (if fitted) and the SAM3U’s internal RTC/RTT/backup registers.
• High-speed 4-bit microSD interface.
• Serial ports, five USART/UARTs, two I2C and one SPI.
• I2S interface for CODECs, DACs etc.
• Up to 86 IO signals, including 16 analog inputs and 10 PWM.
The PCB layout is finished, I've been sitting on it a bit waiting for the Due but TBH I doubt I could stand a redesign if I'm wrong about the processor so I may just go ahead anyway.
Do you have any idea yet about the likely price range? Over $100 or under?
Hopefully well under. I'm working on the BOM right now, just using 1-off prices from Mouser et al it looks like ~$45 including PCB. If I can find a manufacturer I know they can do much better, look at the Olimex prices.
possibly supplementing that with Arduino shield compatible sockets for backward compatibility.
I've lost that now but an adaptor would be easy to make.
In regards to an embedded system the RPi will probably always be nobbled by the OS and limited IO, huge boot time and power requirements. The Gert board is a start and I assume there will be more like it but it will never be a Due, and vice versa. As has been mentioned before I think the two can coexist nicely, the RPi as a high-level human interface/file server/whatever for a Due, and the Due as a low-level real-time IO interface for the RPi.