T-Mobile USA announced that it has begun to offer carrier billing on the Android Market. This will allow customers to buy applications on the Android Market and have their T-Mobile account billed rather than having to use a credit card.
T-Mobile began to push an update on December 9th that will T-Mobile G1 and T-Mobile myTouch 3G users to use the new payment method. It expects that all such users will have the update by early January 2010.
To know whether or not you have received the update, check that the Android Market version is greater than 1660 (Settings > Applications > Manage Applications > Market).
There do not appear to be any plans at this point to offer the same functionality to the Motorola CLIQ and Samsung Behold II which run on Android 1.5.
Now that it appears that carrier billing is not tied to an Android update, hopefully other carriers will soon follow suit and offer the same service.