Photography! That's all anyone ever thinks about when the DSi's dual cameras come up. It's not all that unreasonable, I suppose, considering that most DSiWare we've seen uses the cameras for exactly that. Customize a profile with a photo of yourself, manipulate images via the native camera utilities, use realtime video of yourself to flail wildly in what Nintendo tries to pass off as a Wario Ware title, etc.
The real potential of the DSi's cameras, though, lies beyond the realm of traditional photography. Consider that the cameras are basically the DSi's eyes. It has native visual awareness of its surroundings. If we assume that the software is up to snuff, there are some pretty amazing things that could be done.