New version of Scala plugin uploaded to plugin repository. To use it, download last Diana EAP (http://www.jetbrains.net/confluence/display/IDEADEV/Diana+EAP).
In last plugin version we implemented incremental make for Scala projectsa and improved resolve and completion functionality. Current plugin version doesn't support cross-language compilation via Scala 2.7.2.RC1, so you still have to make cross-language references from Java to Scala order or define appropriate dependencies between project modules.
We're going to support cross-language compilation just after some major compiler bugs in 2.7.2.RC1 will be fixed.
With best regards,