The Sony Blog has news that the MAG open beta is now live. The open beta allows you to experience 64 and 256 player combat after a reasonable download to get the game from the PSN Store:
1. Log on to the PlayStation Store and download the MAG Beta file like you would any other freely-available PSN video or demo (37 MB).
2. Launch the MAG Beta program from your XMB to download a special patch that will restart MAG once the update has been received (another 38 MB).
3. After the reboot, the beta will reconnect and download the actual game data so that you can play (1.8 GB - 2.3 GB depending on your region).