Check that you have the JBoss Tools HTML Editor chosen as the default editor for .xhtml files.
That's bitten me a few times (don't know what caused the switch though).
eclipse has some funny nondeterministic rules for which editor wins the "war" over the various content types.
I fixed the issue by double-checking my org.jboss.wst.xml. This file had a couple of paths pointing to a previous project - and even though this project existed - once I changed the paths to point only to the current project did I get full Seam EL. Oy!
I have same problem. Where is org.jboss.wst.xml or org.jboss.xst.xml file?