In pre-season 2012 the format will be:
ROUND ONE (starting on February 17 and spread across two weekends): All 18 teams divided into six pools of three. There will be a round-robin series of "20-20" games of two 20-minute halves.
ROUNDS TWO AND THREE: Each club be drawn to play a standard four-quarter match on each weekend. Opponents and venues will be known when the draw is released. Each club will be guaranteed at least one game on an official AFL venue.
ROUND FOUR: The two teams with the best win-loss records and percentages will play in the NAB Cup grand final on Friday, March 16 or Saturday, March 17. The other 16 teams will be allocated trial games.
http://www.adelaidenow.com.au/sport/its-user-friendly-nab-cup/story-e6frecj3-1226174610279