An e-mail arrived with a link to a posting from Cornell University on this subject. It seems this is a fairly common sighting. The Cornell posting says, in part,
Each year FeederWatchers report several cases of ‘bald-headed birds,’ mostly Blue Jays and Northern Cardinals. One possible explanation for this phenomenon is an abnormal replacement of feathers (molt). Most bald-headed bird reports occur in summer and fall, which are typical molting times. Many of these strange-looking birds may be juveniles undergoing their first prebasic molt, which produces the first winter adult plumage. For some unknown reason, the bald birds may have dropped all of their head feathers at once. Staggered feather replacement is the normal pattern.The bird doesn't seem any worse for the condition, feeding and flying just like all the other cardinals. However, I have no idea what the girl cardinals think about the way he looks.
Other cases of baldness may result from feather mites or lice, or some environmental or nutritional factor. But no one knows for sure, and the condition has not been well studied. Fortunately, new head feathers grow in within a few weeks.