Due to changes to the TfL website, the high costs of maintaining this service, and the fact the TfL now provide their own API, this site will shut down on May 15th 2014.
I'm a PHP Developer and a while ago I created an XML feed of the current status of the tube lines according to the TFL website by screen scraping their site. I did this in order to power a twitter-based application I was writing at the time. I wrote about it on my blog in order to pass on knowledge of how to scrape pages with PHP but seem to have picked up a huge number of followers who use the provided XML feed for their own applications. It wasn't until I'd received several emails and an Apache access log that was building up at the rate of over 1GB a week that I decided to rethink the XML feed and provide a much better service.
Thus, tubeupdates.com was born. My idea was to have a fully featured REST-ful API that would allow any developer to instantly query what state the London Tube network was in. As a bonus, this would include the Docklands Light Railway and if possible would feature the individual messages that each line has in order to specify exactly why there is a delay or if any stations are closed, etc. A distant dream was to be able to search back through the various status changes over time so that other developers could build applications such as reliability graphs or time checkers.
I'm happy to say that all of the above is now available to any developer via a simple REST-ful interface. In addition, you can request either JSON or XML responses and it's all performed through one simple method!
To start using the API, take a look at the documentation.