http://www.tfl.gov.uk/microsites/itunes/default.aspx

It's only available to customers setting up Auto top-up for the first
time (though I'm not sure how this will be policed with regards to
people who might have more than one registered Oyster card). There are
10,000 promotion codes available, and if they haven't all been dished
out by 29 February '08 already the promotion will end then. And
they'll only give you the code once Auto top-up has actually been set
up on your Oyster card.

An innovative promotion to encourage further take up of Auto top-up,
which is a pretty handy facility.

The limitation of Auto top-up is that it must be ordered online the
day before travel and must specify a particular LU station where one
will start or end a journey, so one must think and plan ahead. I know
several people who would find the Auto top-up facility very useful but
just don't operate like that.

Nonetheless I'd suggest that for some people the expenditure of a bit
of advanced planning, £1 or £1.50 for a single stop Tube journey and a
short walk might be worth it in order for them to set up Auto top-up
on their Oyster cards.