Unless a CFW or other software solution surfaces, there is pretty much no doubting the fact that a flashcard for the 3DS will exist some day. Whether that day be within the months, in a few months, years, or even decades away is yet to be seen. Every relatively popular device gets hacked nowadays, phones, consoles, handhelds, and unless society comes to an end, you can bet that the 3DS will be no different.
It is worth noting that if a software solution arises that allows the execution of homebrew from the SD slot, there would be next-to-no reason to own a flashcard except for piracy.