It would appear that Roku has a native SDK that requires signing paperwork, and Unity can help port with their Union service. Beyond that, I have not found a way to get Unity games onto a Roku. I only have a Roku 2, so I'm a little jealous. I've been seriously considering a 3, but haven't quite talked myself into it.
I still use a Roku 1 but I was just using a Roku 3 over at my in-laws house. The 3 is really smooth and the search function now search across all the streaming sources. It's about time I upgraded.