According to Joystiq:

Quick! Boot up your Metal Gear Online beta, download that one final patch (it doesn’t take too long, we promise) and report for duty. The highly anticipated demo just went live a little earlier than expected (delay notwithstanding) for all territories. You’ll need to create a character (fyi: you won’t be able to change your name, face, or ‘voice’) and pick your initial skill set before firing up a match. After a couple unsuccessful attempts, we got into one but conveniently had no idea what to do so we’re going to recommend you spend some time training.