In the "making our database squeaky-clean" department:
The core schema has default values all over the place that make no sense. For example:
function node_schema() {
$schema['node'] = array(
'fields' => array(
'nid' => array('type' => 'serial', 'unsigned' => TRUE, 'not null' => TRUE),
'vid' => array('type' => 'int', 'unsigned' => TRUE, 'not null' => FALSE, 'default' => 0),
'type' => array('type' => 'varchar', 'length' => 32, 'not null' => TRUE, 'default' => ''),
node.type can never correctly be the empty string. node.vid can never correctly be 0. It is possible that node.vid needs to be 0 or some other temporary value while the node_revisions row is being created (since either node or node_revisions needs to come first, one column will always need a temp value), but that still doesn't mean that a "default value" of 0 is correct.
Having these incorrect default values works fine, of course, but it is semantically misleading. I'm pretty sure many of them are there because of MySQL's rather broken (IMHO) "implicit default values" behavior (http://drupal.org/node/159330) to make otherwise invaid queries work on pgsql.
Cleaning this up is straightforward. Remove the invalid default values, and any INSERT queries that rely on them will pop out during pgsql testing; they are then easy to fix. I suggest that these be considered bugs and fixed for D6, but if someone wants to tell me this is inappropriate during code freeze, I'm fine with pushing this to D7.