Can Scala be used to write GWT applications?
(NOTE: Java/Scala novice here...)
Yes!
It haves Scala+GWT Project, scalagwt. Quick status (my opinion of course): one step before production usage.
Detailed status from scala-lang.org
The Scala to Javascript project is a research project and not yet what we could call a product. You can have fun, but it's not quite at the bet-your-job on it stage yet. There are bugs. We need quite a bit of work on documentation and smoothing the edges.
Github
The Scala+GWT project aims to compile Scala code for the browser via the GWT toolchain. You can fork scalagwt project on github here
Scala + GWT | GoogleDevelopers
See nice presentation and a lot of explanation on Youtube here