Hildon is fully open.
It's just extensions like HildonTimeZoneChooser that Nokia have elected to keep closed. (It's not part of libhildon so I can't really, technically, say it casts a light on Hildon's openness.)

Hildon Desktop, Hildon Home and Hildon Status Menu are open, but they're not as open as they were in Diablo due to their dependence on closed source components in Fremantle.

Anyway, I'm just repeating myself here. Whilst I'm doing this, Nokia are doing what they do best: Producing closed software and staying silent.

At the end of the day, Hildon isn't the issue. Nokia have a lot of closed source components that fall outside of Hildon.