westfw:
what would your first hardware ad-on be?
First prize to whoever reconfigures a Rasberry-pi or similar board to be an "ethernet shield" for an Arduino!
(that may be the best way to get advanced networking features like IPV6, IPSEC, SSL, etc. And it's cheaper than the existing ethernet shield...)
Main usage I see is to connect it to arduino and take the ethernet and sd load to the raspberrie.
Sd can take up to 250 ms and downloading a file is taking long. Moreover starting Ethernet with DHCP on my arduino in my setup takes several seconds. If these delays happen "at a bad moment" my robot is blindfolded and will not respond properly. For a 1000+ euro robot having 2 100 watt engines driving blades at 3000rpm..... 
... I prefer to be on the safe side.
Using the Rapsberry to read/write the serial data from/to arduino; Arduino could send my logging information to raspberry. Raspberrie can process the input (get out the business data) and service the Ethernet and do the SD logging.
At the same time Ethernet actions could be translated to Serial commands send to the Arduino. The nice thing about this is that you can test with the shield or with the serial monitor (so not 2 systems but 1)
This setup would take quite some work from Arduino. It would also make your website/webserver config lots easier as it runs on linux on the raspberry. And finally it would free up some pins for other usage. Maybe I can go back to the UNO 
This would not make it an "Ethernet shield" but a "Smart Serial shield capable of SD and Ethernet"
In short "don't expect me to win the first prize." 8)
Best regards
Jantje
I have been able to order my raspberry today. Expected to ship in 4 weeks.