Steps to reproduce:

• create a new dynamic web project
• enter the name of the project
• select "Default Configuration for JBoss 4.2 runtime"

You will get the following error message:
Seam 1.2 requires JavaServer Faces 1.2 or newer.

The solution is to add the jst.jsf facet to the definition of the defaultFacets ext. point in the org.jboss.ide.eclipse.as.core plugin.