The data will be hosted at the London DataStore, a site set up to give the public access to data from public-sector organisations serving London. A few sets are already up, as well as a beta API which returns the locations of Tube trains heading for a specific station. Which could probably be worked into a mobile app to tell you when to start walking to the station. If they had something like this giving the positions and estimated arrival times of buses (whose travel times are considerably more chaotic than those of trains, and which often run less frequently, especially at night), that would be even more useful. (Some approximation of this facility exists in the LED displays, which are installed at some bus stops and sometimes are operational; a XML feed and a mobile web app would probably be a more cost-effective way of getting this information into the hands of commuters.)
Another thing that would be useful would be an API for the Transport for London Journey Planner; being able to ping a URL, passing an some postcodes or station names, a departure/arrival time and some other constraints, and get back, at your option, a maximum journey time or a list of suitable journeys, in XML or JSON format, would be useful in a lot of applications, from device- or application-specific front ends (i.e., a "take me home from here" mobile app) to ways of calculating the "inconvenience distance" between two points by counting travel time and changes (i.e.,in terms of travel convenience, Stratford is closer to Notting Hill than Stoke Newington, despite being further in geographical terms, as it's a straight trip on the Central Line).
Please keep comments on topic and to the point. Inappropriate comments may be deleted.
Note that markup is stripped from comments; URLs will be automatically converted into links.