Interestingly, it is no longer possible to walk/cycle onto the airport. They are banned from the former pedestrian/cyclist tunnels which have been converted for vehicles.
If that's the case, BAA don't seem to have cottoned on to it - they are still showing cycle access to Terminals 1-3 on their website:
http://www.heathrowairport.com/stati...Motorcycle.pdf
I believe it's only pedestrians that are banned from the tunnel - they can of course travel free through it on local buses.