An anonymous tipster points me to this set of London Travelwatch minutes, which say:
Meeting with First Great Western (FGW) and Passenger Focus to discuss implementation of Oyster pay as you go on FGW. This was on schedule for May 2008 – nine months ahead of most other train operating companies (TOCs).An early rollout was mentioned at another London Travelwatch meeting.
First Great Western recently started accepting Oyster PAYG between Greenford and Ealing Broadway, with an extension to Paddington due soon. This announcement appears to allude to all their London services. It may also include Heathrow Connect as far as Hayes & Harlington, since that portion is technically a First Great Western service.
I also note 9 months from May is February 2009, which is either a miscalculation or they're expecting other companies to miss the advertised January date.