Kind of like Xbox 360 and PS3 themes
During today's Nintendo Direct, themes were confirmed for the 3DS. They'll essentially morph your home menu into whatever you choose, with extra music, sounds, backgrounds, and icons to go along with it.
The ones shown during the Direct include two Mario skins, a Zelda theme, and an Animal Crossing menu. According to Nintendo of Europe these will drop in October -- no pricing scheme is currently announced.
Nintendo of Europe [Twitter]