In the last Nintendo Direct of 2013 Nintendo of Europe confirmed that Super Mario Bros. Deluxe would be part of a Nintendo Network Promotion, where anyone with a Nintendo Network ID registered on a 3DS receives the Game Boy Color game for free. The codes for that promotion are due between now and 28th February.
Despite the title being a free giveaway for millions of gamers in the region it'll also arrive as a standard download on the 3DS eShop. It already has a game page on the store, listing it for a release on 27th February, priced at £4.49 / €4.99; it'll also arrive in Australia on 28th February for AU$6.50.
At the moment there are no equivalent details for North America, while the promotion for a free copy was only in Europe. If you want some early impressions of this eShop version, be sure to check out our preview of the Japanese release.
Thanks to Scary_Old_Lady for the tip.