Details, as provided by Tom and clarified by some research, are: Tom has a bicuspid aortic valve, a minor defect occurring in about one out of 70 people. Tom's has calcified significantly, requiring replacement. He also has a minor aortic aneurysm that would not merit surgical correction by itself, but will be repaired while they are replacing the valve. They will also do a single (so far) bypass.
Tom has been told the mortality rate for this procedure is in the 6% to 8% range.
(Central Dupage Hospital, for those who want to track him down further...)