National Express do it too ... it all seems very obvious, except I'm not sure how it will work in terms of bar code readers at ticket gates; I notice that is starts on limited routes and that's perhaps because of equiptment on the ground issues.  Probably the way of the future - it would certainly be my own preference to book tickets shortly before I left home rather than paying the conductor or at the station.  An understandable ticketting and pricing structure would be needed to stop people ending up with the wrong tickets - no there's a challenge!