Well, someone at Nintendo of America hit the wrong button. It turns out that a number of people that pre-purchased / pre-loaded Paper Mario: Color Splash on the eShop in North America have been able to download the full game early, two weeks ahead of release on 7th October.
The first to spot it was @Electivirus on Twitter:
A thread on NeoGAF includes some others that have successfully downloaded the game, with their preloads unlocking early. Nintendo has now shut off the preload loophole, but nevertheless a number have evidently got their copies.
It's certainly an embarrassing mistake for Nintendo of America; it seems to have only happened in that region, as preloads for the game aren't even an option in Europe, for example.
In any case, some will be enjoying it well ahead of time. No doubt Nintendo will move fast to shut down any early Let's Plays posted online, while someone's going to no doubt be told off for the system / scheduling error.
With thanks to Michael for the heads up.