To work around this problem, I hacked together a python script which moves all your birthdays by any number of days in either direction. You'll need pyobjc installed to make this work, since it uses the Cocoa AddressBook framework. I have Pyobjc version 1.3.6 for python 2.3. If you have entered various birthdays in various time zones, you might find only some birthdays move around. This script won't help you with that. Maybe someone wants to write one that normalises birthdays to a unified timezone.
Put the code in a text file called correctBirthdays and make it executable (chmod 755 correctBirthdays). Run it from Terminal and give it the number of days to shift birthdays by as an argument. For example, the following will move all birthdays back by one day:
./correctBirthdays -1
Needless to say, back up your Address Book before you try this!
[robg adds: I haven't tested this one.]

