I have the same problem. The birthday is correct in Address Book, on iCal on my MBP and on my iPhone, but is one day early on my iPad. I'm syncing via iCloud now.
May be a time zone issue, but I think all devices are set up the same.
I have the same issue on my iPhone 4S ( iOS 5.0.1 ).
My Mac runs Snow Leopard, no iCloud involved.
Some birthdays are off by one day prior to the real birthday-date !?
This applies to dates in the past and future.