BlacknMild, do you listen to the RFN podcast? I asked about this and I think the staff had good points in their answer. The DSi and DSi XL are the same system, so Nintendo has no reason to set up transfer system. The 3DS will be a new system and Nintendo will want customers to buy it, so they likely will have something set up to allow you to transfer DSiWare purchases to the 3DS. You can not assume they don't plan to let you transfer DSiWare games to the 3DS, or the Wii, because there is nothing to indicate that.
Also, we don't know when exactly we will be getting the 3DS, and you can still play your DSi/DSi XL after the 3DS comes out.