Sakurai posted today on Miiverse;
Pic of the day. Users in Japan, you can now download the demo for Super Smash Bros. for Nintendo 3DS! You can pick among 5 fighters with which to play. The stage is Battlefield, and the rules are fixed. There will be a few items and assist trophies available too. You can battle via local wireless, so have fun battling with your friends. For users outside of Japan, stay tuned for more news about the demo for Super Smash Bros. for Nintendo 3DS.
In the demo you can play as Link, Mario, Pikachu, Villager and Mega Man. The only stage is Battlefield and the rules are fixed to timed battles. We're sure this is gonna be more than enough to keep folks happy though. Evidently, the last part of Sakurai's post very much indicates we'll be getting a demo at some point. We'll keep you updated.
Super Smash Bros. For 3DS has a UK release date of October 3rd.