In bug #178, we added a view indicating which nodes are permissible candidates for failover. Use of this view should be added to the "health check" script, where users would be warned if there are any origin nodes for which there are no candidate nodes for failover.
Added patch for this: https://github.com/cbbrowne/slony1-engine/commit/17a721baa30d3442b94baa9088712e0c40f591e5