Well...the point of the SP was not to make a smaller GBA, just a slicker GBA with a frontlight (or was it a backlight? I don't remember squat). Having said that, the SP was more portable.
With the DS...I don't want to miss out on anything if I don't buy a redesigned one. DSpeak, online PictoChat...okay, cool, include them in the new DS, but make them available to the people who actually bought at launch via firmware update or otherwise. I just think it's stupid when companies actually punish their loyal customers who bought systems on launch day and stuff.
Nintendo obviously has no problem with systems competing for the spotlight: the Game Boy Micro was released while the DS was still selling incredibly strong. Of course I hope they learned something from that, seeing as GBM was a failure (though I hadn't actually physically seen the thing till just a couple weeks ago, it's the most retardedly small - in a good way - thing i've ever seen). I think it's best to release a DS redesign summer of next year, extending the DS's life cycle for another year or so, allowing time for GBNext (though I doubt we'll be seeing any more GameBoys; rather, DS-like systems with new innovations).