The excellent Will Sargent has created a Circuit Breaker gem that's quite nice. You can read the docs at rdoc.info. He's released the code (under LGPL) on GitHub.

The other one has actually been out for a couple of months now, but I forgot to blog about it. Scott Vlamnick created a Grails plugin that uses AOP to weave Circuit Breaker functionality as "around" advice. This one can also report its state via JMX. In a particularly nice feature, this plugin supports different configurations in different environments.