Carrier Billing is a nightmare
The only way this should be feasible is if it forces the customer to input security information during purchase confirmation.
That is how it is done in Blackberry App World, after you hit 'purchase' you have to enter a username and password to confirm the purchase. I would be surprised if it was different for the Android Marketplace.
I actually sell Android's to kids (high school) because their parents find out they can't do carrier billing.
How about a simple solution like sending an SMS notification to the parent's handset and email address every time an app purchase is made on an account?
Little Billy might be able to accumulate $50 in downloads before Mommy found out about it, but ...