[No need to use debug_backtrace() however as I have Zend Studio with breakpoints and a call stack view - lucky me ;-]
Have finally tracked down the error in our override template for design/admin/templates/content/edit.tpl. Seems that '{$object.main_parent_node_id}' was causing the fatal DB Transaction error, which is a bit poor.
[I'm guessing main_node_id is not set for new (unpublished) objects and therefore the lookup of main_parent_node_id was causing eZ publish to crash.]