Maybe it's a good idea to also discuss the variants of Arduino like Netduino, Panda, Gadgeteer and others.
Here is one view:
So where do the other microcontroller boards belong? Well, there is the Electronics SE, of course, but there is also the Embedded Programming proposal. That latter is clearly spawned of a feeling that EE.SE isn't ideal for its target group.
If the Arduino proposal were to be diluted to include boards with little in common with the Arduino family, then any dev board or device that incorporates a microcontroller is fair game, and in which case the Arduino proposal isn't about Arduino or its global and very lively community (as it exists independent of StackExchange) any more.
The site scope will/should include all Arduino clones and derivatives that are not too different from the overall platform.
I am not familiar with Panda and Gadgeteer but they appear to be significantly different platforms.
*Everything listed on Arduino.cc will be in scope, which also means Arduino Due and its derivatives will be in scope. However, I do not think we should extend the scope beyond that.