This is most likely a price to value issue for most potential customers.
Currently, for $250, the 3DS doesn't have proportionate value and won't until it starts seeing some legitimately compelling titles that can't be done on the DS whether that's due to the additional processing power, use of 3D effects built into the game design and game play, or other features like Street Pass.
In a slightly worse case scenario, 3DS sales will pick up when Nintendo simply stops making ports of the latest versions of their key IPs for the DS, at which point fans of their games will either skip the games or buy a 3DS.
The pricing issue is always an easy fix however. If production relative to demand is strained, either you sell your product for more or, pass that additional profit to secondary sales (Ebay/hardware flippers) as was the case with the Wii. Nintendo can always drop price once production exceeds demand given the low unit cost of the 3DS.
Given the relatively low difference in production cost between the DS and the 3DS, it does seem a bit less likely that Nintendo will keep the DS in production throughout the entire life cycle of the 3DS for anything other than as a low-cost (to consumers) alternative to maintain a broader market for their games.
