All purchases on board are 'paid' for using the boarding card issued at embarkation and then the account can be settled at the end of the cruise by whatever means you have arranged to pay by (usually a credit card)...you can usually check how the spending is going during the cruise if you ask at the reception desk. Hope this helps. My partner and I will be sailing on the Ventura transatlantic in October 2008.
P&O use the same system as most other cruise companies you register your credit card as you check in but if your paying the bill by cash make sure that you have no on board credits on your account as they seem to take your cash first and any credit not spent is lost?
THANKS.. Buckley and Morris for answering my question I now know what to expect. It would have been a bit embarrasing going on board and not knowing what we were supposed to do.