Welcome to JavaRanch.
There are Scala plug-ins available for NetBeans and Eclipse, the two most popular
Java IDE's. I've tried the Scala plug-in for Eclipse, but had problems with it - strange error messages, basic things like syntax highlighting not working etc. I currently use NetBeans with the Scala plug-in for NetBeans - it works better than Eclipse, but also isn't perfect yet (auto-complete doesn't always work, and occasionally it doesn't show an error for code that doesn't compile).
IDE support will get better in the future. In a few months, Scala 2.8 will be out, and people are working on the Eclipse plug-in, so hopefully it will work better.