I'm sure Nintendo have the hardware pretty well spec'd out but I don't think we'll see a new handheld in the west until at least 2010... Depending on whether Nintendo can increase the hardware sales in Japan or not we may see a new handheld there late 2009 but again I doubt it.. I guess another factor maybe what Sony does but it all depends.
The DS still has so much room for price drops and it seems Nintendo are trying to get much more out of the system such as the talk of plane flight etc..
Also as Iwata stated, sometimes it only takes one single piece of software to drive the sales of a platform so I'm sure Nintendo intend the DS to last a while yet







