Its possible the GBA backwards compatibility may also have been a pleasant coincidence in the hardware design. Maybe not likely, but poissible.
The ARM Processor that serves as the GBA's brain is afterall already part of the hardware, regardless of backwards compatibility or not (I think its the ARM 7, the weaker of the two DS processors?). Adding the backwards compatiblity then may simply have come down to designing an expansion port (which likely would have been included in some shape or form anyway) that accepted GBA carts....I do remember Reggie mentioning that it was also the DS's peripheral expansion port.
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The DS is meant to be a standalone, at least from what I heard of it orignally. Which makes me think that the inclusion of any kinds of backward compatibility is really going to confuse consumers down the line who are going to look at this as the next gameBoy....I know myself and apparently others on this board are confused, and were the ones who stay informed!
At any rate, assuming Nintendo is deadly serious about this "third pillar" concept (Im a sucker for whatever Miyamoto says...if he says its the third pillar Im generally like "Yes, Sir!") I agree with the previously stated opinion that inclusion of the GBC backwards compatibility ( a key GBA-specific feature) only serves to further blur the lines between the two handhelds. As such, it makes sense if they really will practice differentiation of the two units.
I dont know if someone voiced this eariler, but I wonder if Nintendo refuses to call it the next gameboy only because they realize the hardware may be hit or miss....and they do not want to risk tarnishing their most powerful console brand name, the ubiquitous "GameBoy".
-Blackknight131