Platform UI is an ambitious project that provides a simple API for UI elements independent of CSS and JS frameworks, as well as a Theme API for off-the-shelf great looking app UIs. This is not a monolithic "Grails UI" style UI plugin. Its the framework for the basic UI elements used in most web apps, without any specific implementation other than the tags themselves which remove a lot of the boilerplate for people who actually implement the elements. Ever looked at the code behind g:paginate to consider changing the way it writes out the markup? Tip: don't. Platform UI abstracts the rendering for elements like this.
As you might imagine this is quite a big project and I can't do it all alone. The current specimen Bootstrap Theme available is merely a hacked together proof of concept, but the building blocks in Platform UI are pretty much there. There's still some polishing to do in terms of UI element contracts, but that's why this is an RC1.
What we need now is for people to play with this and build some real high quality Themes and UI Sets (the UI elements). I'm trying to make this as easy as possible for people, but there will be more improvements to be made in future for certain. I have written some reasonably comprehensive docs which are available now.
Right now you can also see the "showcase" application source for details of usage and as a harness for testing your own themes on github.
If you clone that code and run it you'll get an ugly UI. If you then install the bootstrap-theme plugin, you'll get a less ugly UI:
grails install-plugin bootstrap-theme
Just run the app again and as if by magic you have a better UI and Theme on the same app. Imagine if that was a professional quality theme and not something hacked together quickly by me as a proof of concept!
The bootstrap-theme and its accompanying bootstrap-ui (UI Set) plugin are intended to be the starting point for future Themes. Bootstrap UI should be reusable for all Bootstrap based themes, but the theme itself is just for demo at the moment.
I'll be launching a little challenge to motivate people to get involved in this very shortly.
If you are interested in Platform UI but cannot play with it at this time, you may be interested in my talk on this subject at GGX 2012