This is nothing new I've been using OS mapping for over 18 months now. I've also put links to it before here on this group, here it is again
http://www.kap.toadstone.com/kapmap/osmap.html Anyone can create or use the Open Space service by registering.
The mapping is dynamic and goes right down to their street level view. I've restricted the field of view only because they allow you a tile amount/day.
In the past year the OS have been recruiting ground data operatives who log and confirm current features. They have also completely mapped from the air the whole of the UK, it started in 2000 I believe. This photographic data is of an extremely high quality/definition and is being used, for example, by local authorities to qualify and confirm infringements of the planning laws/permissions. So anyone thinking they can gradually build and extension or two might do well to think again. I'm sure the OS will gain revenue from this type of data.
As Johnnearcfon has pointed out, the Where's the Path site is an example of the use of Open Space linking the Google API with the OS. Bill Chadwick who developed the application is a very clever chap.
Anyone wanting to use the OS Open Space API can do so here
http://openspace.ordnancesurvey.co.uk/openspace/