From a variety of websites including (all text below from) London Reconnections:

TfL appear to have begun public testing of the new online service for tracking buses via Countdown. The full list of Countdown locations was recently made available on the TfL website recently, giving an overview of where this is likely to prove a useful service.

The service allows users to specify a street or postcode (as with the Journey Planner) and then select bus stops via a map. The live bus departure times (or rather those trackable via Countdown) are then then shown. It's an interesting service and one which will likely prove popular.

The BBC also have an article up about the service (complete with guest quote from Diamond Geezer) and include the following comment from TfL:

A TfL spokesman said: "TfL has commenced user testing of the new 'countdown' system which will provide real bus arrival information for all 19,000 bus stops across London via the web and SMS.

"The existing roadside sign service is currently limited to 2,000 bus stops and this new service covering all buses and all stops will launch fully in the autumn of this year."

It seems likely that an official launch of the service will follow after this initial trial is complete. In the meantime, readers wishing to use it themselves can find it at http://countdown.tfl.gov.uk.