RuliWeb details an alleged interaction between Nintendo customer support in Japan and an individual inquiring about 3DS region locking. The supposed Nintendo Japan support officer specifically spells out, "There are no region codes in DS and DS Lite, but DSi, DSi [XL], and 3DS have region codes. The region codes are installed in each game and the 3DS itself ... you can't play if the region codes are different."
As Tiny Cartridge points out, the DSi and DSi XL both include a form of region locking in that DSi systems can only access their own region's DSi Shop, and DSi-enhanced releases (read: games that use the camera, among other things) are locked to the region in which they're released. The 3DS's region locking sounds like it works the same way: the alleged Nintendo rep spells out, "For example, if you play a Korean 3DS game with a Japanese 3DS system (or vice versa), you won't be able to play at all." Nintendo had yet to respond to request for comment as of publishing.