The first time I've seen this happen on a scheduled basis: when the
District and Piccadilly lines are closed between Acton and Earls Court
for scheduled engineering works over a number of weekends in the next
twelve months, travelcards valid to Central London to Heathrow
(Z1-6)will be accepted on both Heathrow Express and Heathrow Connect
services.

Full details in the TfL press release at:
http://www.tfl.gov.uk/tfl/press-cent...t.asp?prID=398

Has a similar thing happened in the past when there have been
unscheduled closures of the Piccadilly, or would you have paid full
fare on the Heathrow Express?