Payments Purchase Order Number

Can anyone show me how to create a "blank" payment option that users can select and enter a purchase order number during the checkout process. For those clients that have existing net30 payment terms and just need to provide us a PO number in order to get their order placed. Make sense? It doesn't need to say anything other than:

"Purchase Order" and then provide a required text field for them to enter a purchase order number

Is there a "customizable" payment module/extension out there that I have missed which already offers this?

Yep, an actual Purchase Order extension
http://www.opencart.com/index.php?route ... on_id=2711
Check the additional images tab