Have you looked at ScalaTest ?
I’ve not used it, but it comes from Bill Venners and co at Artima, and consequently I suspect it’ll do the job. It doesn’t appear to have IDE integration, however.
This blog entry is a little old, but suggests that TestNG is the best option for testing Scala. TestNG will certainly have IDE integrations.
EDIT: I’ve just realised that I wrote this answer in 2009, and the world has moved on (!). I am currently using ScalaTest, the IDE integration works fine, and I can strongly recommend it. In particular the matcher DSL works very nicely