I'm really impressed by the way Skype handled communications around their massive outage of last week. They promptly communicated about it through a variety of channels (website, blogs, twitter), provided regular updates, acted fast to solve a complex problem and then went on to publish a rather detailed post-mortem post that I encourage you to read: http://blogs.skype.com/en/2010/12/cio_update.html
Compared to Tumblr's communication about its outage, Skype really set the bar high. While Tumblr hardly communicated about the outage and the action undertaken to bring the site back, Skype's CEO himself (or at least someone acting on his behalf) published several posts about the outage while it was still underway.
While we're at this, you might also be interested in this presentation on "the Upside of Downtime":