Delighted to note the Ministry of Defence's decision to publish its new green paper in commentable form, using a restyled version of Steph's Commentariat theme for WordPress.
The MoD have been doing some excellent, if a little underpublicised, work with blogging tools - Defence News and a blog from Afghanistan, both running on a Typepad account; and Blogger-based initiatives from Basra and Helmand; not to mention efforts around YouTube, Facebook and so on. But I think this is their first WordPress-based work.
Looks to be sitting in ultra-cheap hosting space provided by Hampshire-based Justhost.com - £2.95 a month for unlimited disk space and bandwidth; although I think the server itself is in California.
Depending on your definitions, I think that's now a majority of central government departments - or certainly very close to it, anyway - who have run public-facing WordPress-based websites.