Although the problem is fixed in 1.4.4, I just noticed that your testcase doesn't actually test for this bug. The problem was that all issues are returned, no matter what due date they have, if one searches for due_date Feb. 29. A correct testcase would be:

    def testDateLeapYear(self):
        nid = self.db.issue.create(title='spam', status='1',
            deadline=date.Date('2008-02-29'))
        self.assertEquals(str(self.db.issue.get(nid, 'deadline')),
            '2008-02-29.00:00:00')
        self.assertEquals(self.db.issue.filter(None, 
            {'deadline': '2008-02-29'}), [nid])
        self.db.issue.set(nid, deadline=date.Date('2008-03-01'))
        self.assertEquals(str(self.db.issue.get(nid, 'deadline')),
            '2008-03-01.00:00:00')
        self.assertEquals(self.db.issue.filter(None, 
            {'deadline': '2008-02-29'}), [])