So as it turns out, a company named Denco Logic has a patent on this input device that would control very similar to the
Slide Pad Circle Pad on the Nintendo 3DS. I really don't think there's a patent infringement issue here, but funny this is, the company calls their invention the Slidepad. Through some admittedly dodgy research, it doesn't seem they (or anyone) ever trademarked the name Slidepad in the United States. When I first heard that they changed the name to Circle Pad, my theory was that "Slide Pad" could be mistaken as a reference to the touch-screen, where someone unfamiliar with all or a part of the 3DS's technologies could connect how you slide your finger or a stylus on the screen with how many people would refer to a writing surface as a "pad" (see: Apple iPad).
But because of this small company and patent, I think it's a combination of the misinterpretation and existing product that caused them to change it's name.
My thanks to YAMI_ANUBIS_X for inspiring this article.
Denco Logic's site here.