I think it is more a privacy issue than anything else. Most people don't use their real birthdate when signing up anyway, so it may also cause some confusion and possibly accusations of being untruthful. I guess you could say that birthdays fall under the 'personal information' category.
What I would like to see though, is an option that sends an active user a birthday greeting and a little gift automatically on their birthday. That would be pretty awesome, right? :D
Also, I think we need the ability to send birthday cards that allow us to write a personalized message on them. Or at least a choice of messages from a drop-down menu.