It shows up validation errors and what not, but now when I add like this in an action method:
FacesContext.getCurrentInstance().addMessage(componentId, new FacesMessage(severity, rawMsg, null));
I think Simon is referring to
http://www.jsf-faq.com/faqs/faces-messages.html#110
I have my generic (not component targeted) messages in a request-scoped messagebean with two lists for messages and errors. But targetting a component from your actions could be a little tricky. Perhaps it could be done with facelets or some other trimmed viewhandler(?)
