From Halopedia, the Halo wiki
Bungie Pro is a 6-month subscription service that can be purchased over Xbox Live for 320 Microsoft points. Bungie Pro enhances a Halo 3 player's File Share, increasing its capacity from six slots and 25MB to twenty-four slots and 250MB. It was confirmed on one of Bungie.net's Weekly Update that Halo: Reach will include Halo 3's File Share system including Bungie Pro.
As previously stated, a Bungie Pro subscription increases the capacity of a player's File Share. The size increase lasts until the subscription expires. Upon expiration, a player's File Share reverts to normal only if their extra slots are all unused. If a player continues to use their extra storage space after their subscription expires, then their File Share will not be reverted until they reduce it to meet the limitations of a normal Share, but the player will not be able to upload new content until their subscription is renewed or their Share is reverted.
A Bungie.net news post describing the service relied primarily on a description written by Roger Wolfson, "a super genius working on all manners of Database and information tracking for Bungie Studios". That description, with some unnecessary content removed, has been reproduced below.
Bungie stated that as of September 15, 2011, Bungie Pro will no longer be available for purchase. Those with subscriptions that end after the termination date will get to keep their Bungie Pro features, the 24 slots and nameplate, forever. Render minutes are no longer available as Bungie turned all Halo: Reach support over to 343 Industries.