I'm not sure how debit cards work outside of New Zealand, however here in New Zealand it is impossible to use a debit card without entering in a PIN code to authenticate yourself to the terminal. In other words signing off a debit card transaction isn't possible. So if a bank were to post a pre-activated card out to a customer, there's no security risk since the customer would have picked a PIN code when they originally signed up for the card in the branch.
I'm with the ANZ Bank here in New Zealand and they post out replacement debit cards with the PIN already assigned to the card. As far as I know the PIN isn't loaded on the card itself. The bank just adds a record on their side saying this new card has the same PIN as the previous card. No security risk since only me and the bank's debit card system knows my PIN number. Someone stealing my card would have to guess my PIN.
Perhaps this is not the case outside of NZ. However I personally think the 'signature required' type of transaction should be withdrawn, leaving PIN as the only authentication method. We in New Zealand have managed to do without signature transactions for debit card transactions since EFTPOS was introduced in the 1980's.
Signature authentication is only allowed for Visa/MC/Amex credit transactions since NZ based merchants have to accept foreign cards which allow signature required transactions. So basically we only allow signature required transactions due to having to keep that security loophole open to allow foreigners to pay for goods and services in NZ.