PSN update, a "Full Game Trial" of MAG will be available to download in the Plus section of the PlayStation Store. However, unlike most Plus content, you'll have access to this content for as long as your membership is active.
Of course, there's a catch: This is still only a game trial. Although you'll be able to play with and against actual owners of the game online for as long as you want, the full-game trial has an artificial level cap. You'll only be able to advance your MAG character up to level 8, enough to unlock access to the game's signature 256-player mode. Any experience you earn after level 8 will go unrewarded until you purchase the full digital license for $30.
The trial's level cap means you won't be able to become a squad leader or change your faction either, but if you want to keep playing for free, simply put an artificial cap on your gameplay goals, too: Become the greatest level-8 soldier of all time.