Describing the Wii U image above, Sakurai wrote, "Pic of the day. Looking at the number of days we have left for development, it would be an impossible task to create this… That's what I told my staff. But thanks to the determination of her female designer, these Zero Suit outfits got completed in time. From the ending of Metroid: Zero Mission, here's Samus in shorts!"
As Sakurai points out, the suit is a callback to the an outfit you see Samus wearing after you beat Metroid: Zero Mission and Metroid Fusion under specific time and item conditions. And the same outfit will be available in the 3DS version of the game.
Skimpy alternate costumes have been around since the first Metroid game, where if you got to the end in less than an hour, Samus would show up after the credits in a two-piece bikini.