I'm working through my buzzword complaint application which naturally includes Cloud computing (Google App Engine), GWT, Scala and the rest of social Web2.0 hyped BS. Wow, that was a keyword loaded sentence ;-)
GWT has a nasty exception to my experience so far that you can replace any Java code with Scala. When you try to have a GWT EntryPoint be implemented in Scala you get this GWT compilation error
Checking if all subconditions are true <all>
[ERROR] Unable to find type 'com.newspipes.client.Newspipes'
[ERROR] Hint: Check that the type name 'com.newspipes.client.Newspipes' is really what you meant
[ERROR] Hint: Check that your classpath includes all required source roots