That's a good idea. My brother's forum does that. That way there isn't different topics for everyone's birthday. It's a better way to celebrate. Then again, there is the topic "What are you celebrating today" which is almost the same thing.
At one of the other forums I visit they have also put up a calenda with everyone's birthdays. Well the people who want to give it. But no one can reply or add to it except for the moderators and adminastrators. But first you would need to have a topic so people can add their birthdays and after a while when there are a few then put in the calenda. Having the calenda at the begining all the time means that people don't have to go through pages of topics just to see who is having a birthday if noone has posted for awhile. But it would be a good idea to keep the birthday topic at the begining for any new memebers who wish to add their birthday