Oh and tfl have been a bit arsey in the past with third party applications using this data. See here, for eg: http://mbarclay.net/?p=842 ...so please be nice and don't hammer the API too hard

Also, I'm using the "user" frontend, but might be better to use the ultralite one so save on bandwidth

Some URL examples (route from two random postcodes, for example -- change type_origin to be address, OS grid reference, or anything you want)
Mobile website:
Full XML
ultralite XML

It has always been on my TODO list to do a proper journeyplanner, but here's hoping someone else can take this on