One surprise in the 2012 fare revision is that the Piccadilly line
joins the Heathrow Free Travel Area.
See
http://www.tfl.gov.uk/tickets/20689....athrow-airport
"Journeys solely between Heathrow Terminals 123, Heathrow Terminal 4
and Heathrow Terminal 5 will be free for all Oyster card and Oyster
photocard holders"