It's on the official blog, albeit below the fold
http://newsroom.fb.com/News/558/Update-to-Messaging-and-a-Te...> Today we’re starting a small experiment to test the usefulness of economic signals to determine relevance. This test will give a small number of people the option to pay to have a message routed to the Inbox rather than the Other folder of a recipient that they are not connected with.
> This message routing feature is only for personal messages between individuals in the U.S. In this test, the number of messages a person can have routed from their Other folder to their Inbox will be limited to a maximum of one per week.