I upgraded my Mac Book to 10.5.2 today. Everything went swimmingly, but I noticed that everyone I know had suddenly moved their birthday to Wednesday the 13th at 15:00 EST–according to my calendar anyway.
My fix for this was to go into the iCal preferences and uncheck the Show Birthdays Calendar checkbox, then recheck it. This put everybody’s birthday back where it was supposed to be.
Update 2008-05-29: This birthday calendar weirdness happened again with the 10.5.3 update. Although, only three birthdays got screwed up. Once again, unchecking and then checking the Show Birthdays Calendar fixed the calendars.