As the model for the e4 workbench stabilizes we're back working hard on the compatibility layer. Right now it consists of the gutted org.eclipse.ui.workbench plugin. The idea is to support the API we have in org.eclipse.workbench, but based on the e4 workbench model and e4 services, instead of the mass of internal code in parts, perspectives, and presentations.
We're taking a 2 pronged approach. Creating an e4 IDE application and slowly adding useful views and actions, seeing what is needed to bring them up. We want to support a useful number of views (like the Project Explorer and Problems view) sooner rather than later.
We're also running the org.eclipse.ui.tests.api.ApiTestSuite (after cleaning up internal references in the tests themselves with the aid of a tweaklet). ApiTestSuite covers the most common scenarios (opening and closing windows, perspectives, views, and editors), and supporting our API is a good way to help 3.x plugins run on e4 with the compatibility layer.
There's always a lot to do, so if you are interested please check out http://wiki.eclipse.org/E4/Compatibility/Running_the_compatibility_layer. You can also post to the e4-dev mailing list, or join us on irc at irc://freenode.net/#eclipse-e4
