I don’t see how it is. Depends on who you’re telling I guess. I don’t think random strangers need to know, but if you’re telling friends or people on a forum site, I don’t think it’s wrong. That doesn’t mean an entire Fluther question is necessary to do that (not that anyone has done that before [maybe?], but I’ve seen people start threads on other sites to announce their birthday and some people saw it as blatant attention-whoring…)
Or do what I did a couple years ago and change your birthday on Facebook to March 6th even though that’s not your birthday, just to see who will respond so you can find out who your “true friends” are :P
Happy Birthday, bt dubs :)