In the Q&A session at Nintendo's Investor Meeting yesterday, Iwata confirmed that Super Smash Bros. would feature online play.

Satoru Iwata: "Smash Bros. will have online play to meet customers' demands."

Neither platform was mentioned so it's safe to assume that both the Wii U & 3DS versions will an online multiplayer mode.

The translated summary was provided by NeoGAF's Cheesemeister.