Although the NY Times reports that Bush Sr. and Jr. talk (in private) on the phone every day, this article from the London Times reports Bush Sr. being openly critical of his son when speaking in public:
The first President Bush has told his son that hopes of peace in the Middle East would be ruined if a war with Iraq were not backed by international unity.
… He also urged the President to resist his tendency to bear grudges, advising his son to bridge the rift between the United States, France and Germany. …Mr Bush Sr even came close to conceding that opponents of his son’s case against President Saddam Hussein … have legitimate cause for concern.
Thanks to BoingBoing for the heads-up.