I've been thinking and using more Scala IDEs.  Here are some additional thoughts.

NetBeans 7.0 with the latest Scala plugin works really, really well for Maven-based Scala projects.  NetBeans has the best (most accurate and timely) error reporting as well as excellent code navigation.  It works with Scala 2.8.x.  It's just really, nice.  I haven't been able to get it working with Ant or SBT projects, but I haven't tried that hard.

IntelliJ 10.5 RC2 is very nice as well.  It's still got some error reporting issues (lack of accurate errors... sometimes too many, sometimes too few).  It works nicely with Ant-based and SBT based projects.  I'm using it today to do Lift coding (not a Lift app, but the framework itself.)

The very latest Eclipse Scala plugin.  I say nothing because I have nothing nice to say and I spent a good 2 hours with it this week.