As per this email from the Adwords Team, Google is beta testing a new Adwords billing interface & payment option in some new accounts.
Google aren't up to date on many things, but talking of billing - you should be able to switch from postpay to prepay and vice versa, if you so wish. This has always been a 'no brainer' but Google still haven't grasped that. It makes you realise that even companies the size of Google still have a hell of a lot to impove on.