My wife has just upgraded to Panther (on my advice) and in iCal, many of the birthdays have moved forward by one day!
What can this be? Another possibly relevant fact is that this problem might have also co-incided with the clocks changing a couple of weeks ago. There were a number of problems reported in the Spring when the clocks went forward by one hour.
I can find no other reports of this on these forums - any ideas?