Confirmed! This is pretty damn amazing.
I really think his entire tumblr post is worth reading, but Michael Dante DiMartino says:
The main themes of the Avatar universe have always revolved around equality, justice, acceptance, tolerance, and balancing differing worldviews. In subtle and maybe not so subtle ways, Avatar and Legend of Korra have dealt with difficult subjects such as genocide, child abuse, deaths of loved ones, and post traumatic stress.
Our intention with the last scene was to make it as clear as possible that yes, Korra and Asami have romantic feelings for each other. The moment where they enter the spirit portal symbolizes their evolution from being friends to being a couple. Many news outlets, bloggers, and fans picked up on this and didn't find it ambiguous.