From London TravelWatch Board Meeting Agenda for Meeting on 16th
December 2008 - report for item 10, 'Fares Changes in January 2009' -
paragraph 2.6.....

"These changes (rises on National Rail zonal fares) are an interim
arrangement until 20 September 2009 when Oyster Pay as you Go is due
to be introduced across the National Rail Network in London. At this
point a fare structure based on the peak/off-peak principle that is
being introduced on the Underground, Overground & DLR will be put in
place. However, the levels of fare will differ from those on the TfL
modes and when this is introduced some passengers who currently buy
daily, point-to-point or Travelcard season tickets may be better off
using Oyster Pay-as-you-Go."