Very cool talk about the unit testing capabilities for JavaScript in script.aculo.us by Thomas Fuchs: By means of a Rails plugin, one can integrate JS unit tests into a rails project. Executing the tests actually fires up Webrick and the browsers you have available and executes the tests in them. Someone asked the obvious question about whether using Rhino might be another option, but Thomas pointed out that cross-browser testing is one of the main goals of the framework.

Update: Links added, slides are available.