The youtube video announcement.
I was trying to do it like announcing a new born... congratulations, it's a girl... but it didn't really work.
I think the writers won't be able to resist looking at some old relationships under the new gender dynamic, as there have been quite a few openly gay and lesbian characters in Dr Who, Torchwood and Class (which had an actual gay sex scene), and this would technically be the first transgender character as well.
Alex Kingston seems to be happy with the news, so even though last year's Christmas Story was supposed to be their last time together, I think the writers would love to see how River would treat her husband, errr, wife.