The US-based branch of PlayStation Network is launching a promotional offer for October in which you’ll receive $20 USD in credit for spending a total of $100 USD during the month. It’s a one-off deal, so you won’t receive (say) $60 USD for going mad and spending $300 USD; but anything in the PlayStation Store counts.
If you’ve already pre-ordered something from the Day 1 Digital selection, that counts towards the $100 USD total.
The $20 USD in credit will be delivered at some point during November, via the email account you used to set up your PlayStation Network ID. So make sure you still have access to that.
It’s not clear if this offer extends outside the US, but since it doesn’t appear to be mentioned on the European blog I’d hazard a guess at it being US only.
Source: PlayStation Blog