Me and a friend were discussing the 3DS and I mentioned how I was nervous about Nintendo releasing too many hardware systems at one time. We have the DS fat, Lite, DSi and DSi XL and eventually the 3DS. I stated that Nintendo might be following in the foot steps of SEGA in regards to the Genesis, CD, Saturn, 32X, and game gear. Where they release too much at one time, oversaturate the market with devices that actually interfere with each others sales and eventually lead to the companies down fall.
Second, I think of the DSi as more of an experiment to show Nintendo what to do with the 3DS than an actual successor to the DS lite. The 3DS will most likely be a beefed up version of the DSi. So I would just use the DSi as a template for the 3DS and add what ever hard ware improvements and bells and whistles that Nintendo will add to the system.