For the past week or so, Oyster pre-pay has worked on every bus I've
tried it with. The reader light goes from amber to green, 70p is
deducted from the credit on the card and the remaining balance is
displayed.

So despite the anticipated launch being at least a month away, the
service is ready to use.

Interestingly, prior to this, it was possible to use Oyster to get
free travel on most DOO services. Throughout much of January, if you
attempted to use Oyster prepay on a bus, the reader light would go
from amber to red, credit was not deducted, and a rather stupid error
message appeared on the fare machine: "OYSTER IS VALID FOR TRAVEL
THROUGHOUT LONDON BUSES". Despite the red light, the contradictory
error message meant that most bus drivers would let you on for free.