Google has updated the Play Support accepted payment methods page with a few new carrier billing options, including billing for subscribers of 3 in Hong Kong, Ireland, and Sweden. Wind Italy has apparently received carrier billing as an option, as well.
Oddly, Telenor of Sweden has been removed from the list as well, though we've not been able to confirm that the feature has actually been disabled. Depending on your provider, you may need to manually enable carrier billing as an option on your account page, as some 3 users are reporting.
For countries where credit card penetration remains low because of tight consumer credit, carrier billing is much-desired as a feature on smartphones. It's also a bit easier than entering a credit or gift card into Play. If you're on any of the aforementioned four carriers, let us know if carrier billing has been enabled for you.