I posted this (http://news.ycombinator.com/item?id=1236781) with a link to The Guardian article (http://www.guardian.co.uk/news/datablog/2010/apr/02/ordnance...). The Guardian have long run a campaign to get the UK to open their data, and the OS is a really big win. Prior to this, post-office data was very expensive to license (despite the information having been gathered with UK tax payer money)

You can browse the data available and download the CSVs directly here: http://data.gov.uk/data/publicbody/Ordnance%2520Survey

Data includes full postcode lookup:

http://data.gov.uk/dataset/os-code-point-open

And all transit (bus stops, train stations):

http://data.gov.uk/dataset/naptan

Am I right in thinking that this is post code <-> lat/long, and not post code <-> address which Royal Mail still keep under wraps?

Yes. There's info on exactly what everything is here:

http://wiki.openstreetmap.org/wiki/Ordnance_Survey_Opendata#...

They're using Amazon EC2 to host it which is pretty notable for a government agency.