A good number of projects have started using JBehave as their functional automation test framework. I believe it's a good framework that encourages concise specification of system behaviour. Plus the Given...When...Then... format is something very familiar to folks on ThoughtWorks projects as this is how we represent acceptance criteria of user stories.
Something that I believe can slow one down when using JBehave is navigating between .story files that are plain text files and their Java counterparts (JUnit runnable classes and Java classes containing step implementations). This navigation is frequently required to run tests from the IDE and to make incremental changes to step implementations... many times it helps to simply ensure that the step text in a .story matches the annotation string on the Java side!
I've made an attempt to make this navigation a little easier for the IntelliJ IDEA IDE in the form of a plugin named JBehave BDD Plugin.
More details are mentioned here: http://bitbucket.org/amanking/jbehave-bdd-plugin
The plugin is available for installation from within IntelliJ IDEA 9.x via the Plugins settings (Available tab).
Currently the plugin is a bit limited in features but is still pretty usable. If you give it a try, please provide feedback.
Of course if you are interested in a similar tool but well-supported with IDE features like navigation, refactoring, etc already built in, you should look at Twist