Bungie Pro

Bungie Pro is a 12-month subscription service that can be purchased over Xbox Live for 750 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 is not known whether or not the service will carry over to future Bungie games.

Background
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.

Bungie Description
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.