Harmattan's UI is not open source as well. The core is, though, and the distro containing the open-source components is called Mer. Mer is the underpinning of Sailfish.

If Jolla's claim that they can have Sailfish running on a brand new hardware platform after one day of work is true, blows cleanly out of the water one of the most infamous claims of Steven Elop, that Harmattan could not easily be adapted to new hardware. Harmattan had several Nokia-specific adaptations that made it more dificult to adapt than Mer, but what's the problem in taking one month to adapt to a new platform? Android takes six months or more!

I'm seeing the UI demo (last 10 minutes of the Slush keynote) and will comment on that when I have an opinion.

So far, I do not like the "swipe" being a dissolve instead of a card (a LOT less intuitive than the N9) and too much spindly thin text like WP8.