Downloaded content for the PS3, XBOX 360, or even the iPhone is easily transferable because it is linked to the actual consumer, not the device.
Nintendo on the other hand thinks you need to pay for the same apps/games twice if you want to port over to your new console. Unlike the PS3 or XBOX360 which is linked to the consumer, Nintendo's are linked to the actual device.
Remember this when you think about upgrading to the new black Wii coming out or the 3D DSi...