One strategy to get completion and validation working for facelets on eclipse: use WTP's xml editor for the xhtml files and assign xsd schemas for the JSF and Facelets namespaces. The author of this post provides the xsd files and shows how to set them up in Eclipse's XML catalog.
The problem is that, at least on Eclipse 3.3, it doesn't seem to work.
Another approach for getting IDE support for Facelets on Eclipse: use a <jsp> tag instead of an <html> one for the root of the pages. One hell of a kludge...
Have you discovered the Display view in the Eclipse debugger? It is my personal favorite. In it, you can execute any Java code in the context of a running debugger.