A big part of Tizen will be to have a framework and the corresponding SDK to support HTML5-WAC applications. Native applications development should also be supported through the usage of the EFL (Enlightenment Foundation Libraries—http://en.wikipedia.org/wiki/Enlightenment_Foundation_Libraries) with the SDK. So we could suppose that the reference UI of the system will also be based on the EFL.
Of course this all pretty unconfirmed, but still, if you have ever seen what a performance EFL delivers on rather poor hardware (I am talking of the Openmoko FreeRunner here, of experiences I made years ago, so it has likely matured since thanks to Samsungs founding), this sounds promising. BTW, there will be a Tizen event in China in early December, so we may be knowing more soon.