As of April 2014 you can preorder the DRM-free Frontiers on the game's website using the Humble Bundle widget.
You can also get a Steam key if you preorder there.
(obviously not before the game is released so don't hold your breath)
There are no extra perks with that beyond the warm fuzzy feeling of having supported the project.
There isn't even a potato-shaped Team Fortress hat.
You'll find more information in Kickstarter Update #50 .