You can take a look on org.eclipse.gmf.ecore.edit.policies.OpenDiagramEditPolicy
from org.eclipse.gmf.ecore.editor plugin (available in GMF CVS repository:
/cvsroot/technology co org.eclipse.gmf/examples/org.eclipse.gmf.ecore.editor).
OpenDiagramCommand existing in this class is a writable command and will
be executed on double-clicking to the diagram node.