I think a birthday luck buff would be cool, that increased positive REs. Personally (and this is humbug attitude, I know) I'd love a birthday page on the main news and to do away with the special user icons all together. As someone who stalks things all the time... it really bugs me when people who are quiet 95% of the time suddenly become chatter boxes on their birthday just to get attention/gifts. Plus I love giving out random gifts to people, and having a page full of birthday someones would be a nice easy list of targets. I'd also fully support notifications for friends on the birthday. Because I'm always super paranoid I'm going to miss one of my special people's special days.
As a strange sort of side note, I also think it would be cool to have some kind of "RE"/notification/cute story thing for pets on their birthdays. Maybe full huger/happiness buffs/free training for that day.
Yes please for Birthday perks. Today is my birthday and I would have liked something. I have no ideas.