Here's a small AppleScript I wrote to create a copy of the Birthdays auto-calendar in iCal, resetting the start dates of any early birthdays to 1980. If you save the script to ~/Library -> Scripts -> Applications -> iCal, and install the AppleScript menu item, you'll be able to create and update your syncing calendar with ease. All you need then do is select the correct calendar for syncing in iSync.
[robg adds: I haven't tested this one.]

