The PlayStation All-Stars Battle Royale public beta starts on September 11, lasting one week to September 18. Anyone interested in being a part of the frantic action can sign up on the PSN beta site right now.
Players chosen will be notified via email, get a voucher code for the download on either PS3 or the PS Vita, and receive "test night information." Presumably, developer SuperBot will aim to test specific heroes, modes, or stages, so it'll direct players that way.
If you don't make this early cut, you'll have to wait until the full game arrives on November 20.