The Tizen SDK provides support for Windows and Ubuntu while the source-code to it is available so it could in fact be ported to other platforms.
The widget-oriented Tizen Web UI framework is an extension of jQuery Mobile. The Tizen Web API has picked up new features as well for doing matters like accessing an alarm, accessing media content, retrieving system information, and more.
Tizen isn't yet ready for use in a production capacity, but they're still targeting a Q2'2012 release.
Find out more from this week's blog posting.