This is a semi-painful-semi-manual process at the moment, so will not be automagically updated. If you want write rights to manage the calendar, let me know.
Yeah, so it turns out that @xntrek did this Twitter/Tumblr birthdays-as-google-calendar thing several weeks ago, and even emailed me to tell me about it, and I somehow completely missed it.
So I deleted my version (which had all of two birthdays on it) and suggest that you use his instead.
In the immortal words of H.J Simpson: D’oh.
@TJ has been set up as a manager of the calendar. So feel free to contact either of us to be added as an editor.