A happy event is only truly complete when it is universally shared and celebrated. So when your friend and colleague announces a happy event, it is vital not just to congratulate him/her/them, but also to share your joy as widely as you can.
Firstly, be sure to select the right means of communication. Congratulations expressed in such an ephemeral medium as IRC, or (heaven forbid) the human voice, are lost so quickly, and anyone who is not present is denied the opportunity to share. Likewise an old-fashioned card or private email, text message or similar is a selfish act of not sharing. A message on the principal’s blog is almost as bad: it’s so rude and impersonal to make people come to your comment, when you could be sending it to them (and besides, some people might have already read the blog and forget to return to see your congratulations).
The right thing to do of course is to post your congratulations to big and busy mailinglists. The medium is suitably personal, with every happy subscriber getting an individual copy of your joy. These days of course everyone has unlimited storage and bandwidth, so be generous. And don’t forget that even in a techie community, not everyone will read your message on a ‘phone or even a laptop. It takes quite a lot of “Re: Happy Event” messages to fill a big screen with joy, and it’s not a truly happy community if there are so few messages as to leave other topics visible on someone’s mailer!
Oh, and congrats to DrB.