It will be Playstation Plus at least for the first rounds. In mid October they're starting with Playstation Plus members only. Don't know if they're expanding from there. However, there's a catch. Not everyone on Playstation Plus will be allowed in the beta. You have to do the following:
1) Be a Playstation Plus member.
2) Download a special Killzone 3 theme once it's available, this'll enter you in the beta pool.
3) Be chosen randomly from the beta pool to participate. Only 5000 US participants will be chosen.