The Due will continue to work with all Arduino shields — add-on boards and circuitry like motion sensors and LED light arrays — that conform to the official Arduino Revision 3 layout. However, the Due operates at 3.3V whereas AVR-based Arduinos operate at 5V, meaning some third-party shields that don’t follow the R3 specs to the letter may not be compatible, depending on their voltages. http://www.wired.com/design/2012/10/arduino-due/
I looked all over the arduino site and didn't find this shield specification. The reference section is only about the language. The pages for official Arduino shields don't seem to mention, far less link to, these specifications.
Does anyone know where they are? If it is important to comply with them, shouldn't they at least be discoverable?