I like it when people tell me happy birthday (on my birthday
), but telling people that it is your birthday or that your birthday is approaching is like saying 'I expect you to wish me a happy birthday on this day, it is now your duty' which completly destroys the happy feeling received from someone telling you happy birthday. The same goes for chain-"Happy Birthday"'s. If its not sincere I'd rather not hear it at all (not to be pretentious, its just meaningless for both parties).
Disclaimer: I'm referring only to a group of peers that spend enough time with each other to actually know each others birthdays. I don't think the above applies in a professional or impersonal environment.