According to William Dangerfield on Twitter 'apparently Oyster has been 'declined' at Harpenden station.'

Reason being...

The alternative system, ITSO, is scheduled to be introduced in St Albans - however, not until 2014.

The Government-backed travelcard scheme is already in use in other parts of the country and the Department of Transport wants to extend it to the south east. Travellers will be able to use their cards on trains and buses across the UK.