following expressions worked well under 2.6.9:

value="container.messages.getMessage(getBlockId())"

or

value="container.messages.getMessage(blockId)"

java code:

public abstract String getBlockId();

public Block getSelectedBlock()
{
   String selectedId = getSelectedBlockId();
   if (selectedId == null)
      selectedId = (String)getBlockIds().get(0);
   return (Block) getContainer().getComponent(selectedId);
}

return the following error: "setter body: null " in detail:

Unable to parse OGNL expression 'container.messages.getMessage(getBlockId())': Error compiling expression on object $TabPanel_41@778611c5[Home/$TabPanel] with expression node container.messages.getMessage(getBlockId()) getter body: { return ((org.apache.hivemind.impl.AbstractMessages)(($TabPanel_41)$2).getContainer()).getMessages()).getMessage(((org.apache.tapestry.AbstractComponent)(($TabPanel_41)$2).getBlockId());} setter body: null