Terraria is on the 3DS eShop already, having arrived in the most recent update. It had been long anticipated, especially by those Nintendo fans seeking a version on the go - the Wii U version hasn't made it out yet.
A member of the development team has been clarifying details, however, and the good news for Wii U owners or those that'd prefer the physical retail version of the 3DS entry is that both are planned for 'early 2016'. Some interesting specifics were given on the current 3DS version, useful for those concerned with version numbers and included features:
Terraria 3DS includes all the content from 1.2 with the exception of Dyes which are excluded on technical grounds. To make up for that, we've added Minecarts, Fishing and a few 1.3 UI options like the ability to quick stack to a chest. The 3DS version also features a completely overhauled interface, making full use of the two screens. You can cycle the bottom screen between an intuitive touch to build interface; a world map; or as a quick-select inventory bar. It's the most intuitive Terraria has ever been on console, and we've included a few surprises for the Nintendo fanbase.
At launch, there is a single world size of 1750×900. You can save up to three Worlds and three Characters per console. This is as large as the original 3DS can handle. We are investigating the possibility of introducing larger worlds as a New 3DS-exclusive feature in future, but for launch we felt it was important to start everyone off on a level playing field.
As was previously known, it was reiterated that there's no autostereoscopic 3D, but there is better performance on the New 3DS models; the game runs at 60fps on those systems and 30fps on the original hardware. The developer is also "planning to support both Nintendo versions post-launch", though unsurprisingly has confirmed there'll be no 3DS / Wii U crossplay due to the limits of the world size on the portable.
We're still working on our review, in the meantime, but if you've already picked this up on 3DS let us know what you think of it in the comments.