We are redoing the OpenMRS UI from scratch for the OpenMRS 2.0 release. This gives us a one-time-only chance to choose a new platform also.
We decided to build a custom framework on top of Spring MVC, using groovy templates as the view technology. The framework is under active development, but it's "ready to use" in a pre-alpha state. You can see the latest documentation at: Reference Application 2.x Developer Documentation
We haven't done much work on deciding what the content of the UI should look like, but you can see a bit of preliminary work here:
Some historic background about choosing the technology: