Kent Beck and David Saff have released JUnit 4.0. JUnit 4 takes advantage of Java 5 features like annotations, varargs, and generics to simplify unit testing still further. If you're committed to Java 5, then JUnit 4 is big leap forward. Unfortunately while JUnit 4 is backward and forwards compatible with JUnit 3 test suites and test runners, it's completely incompatible with Java 1.4 and earlier so I'm afraid most of us will be sticking with JUnit 3 for some years to come.
The JUnit web site has not yet been updated. Until it is, the article I wrote for developerWorks is probably the best introduction to this version. I think it's mostly still accurate.