T-Mobile Forums moderator, Dominic, posting in T-Mobile Forums:
Beginning December 9th, customers on post-paid accounts with devices running Android 1.6, such as the T-Mobile G1 and T-Mobile myTouch 3G, will begin to receive an update to the Android Market application to allow for purchases to be charged directly to their T-Mobile account. This allows customers to purchase paid applications without entering credit card information.
This could be a big deal. Separating the actual transaction from the download of Android apps could cause a big jump in sales.