It seems that delete() called on an empty Doctrine_Query does the deletion of all records but does not return the number of affected rows. Here is a test case:

function createTestData()
{
  // Reinitialize all
  Doctrine_Query::create()->delete()->from('Article')->execute();
  $article1 = new Article();
  $article1->setTitle('foo2');
  $article1->save();
  $article2 = new Article();
  $article2->setTitle('foo3');
  $article2->save();
}

// Create test data and check count
createTestData();
echo Doctrine_Query::create()->from('Article')->count();
=> 2

// Delete all with no WHERE clause
$nbDeleted = Doctrine_Query::create()->
  delete()->
  from('Article')->
  execute();
echo $nbDeleted
=> 0 // WRONG
// check count
echo Doctrine_Query::create()->from('Article')->count();
=> 0 // But everything is deleted, as requested

// Create test data and check count
createTestData();
echo Doctrine_Query::create()->from('Article')->count();
=> 2

// Delete some with one WHERE clause
$nbDeleted = Doctrine_Query::create()->
  delete()->
  from('Article')->
  where('Title = ?', array('foo2'))->
  execute();
echo $nbDeleted
=> 1 // OK
// check count
echo Doctrine_Query::create()->from('Article')->count();
=> 1 // OK

// Create test data and check count
createTestData();
echo Doctrine_Query::create()->from('Article')->count();
=> 2

// Delete all with one always true WHERE clause
$nbDeleted = Doctrine_Query::create()->
  delete()->
  from('Article')->
  where('1 = 1')->
  execute();
echo $nbDeleted
=> 2 // OK
// check count
echo Doctrine_Query::create()->from('Article')->count();
=> 0 // OK