A post on the Battlefield Heroes official website indicates that you can sign up again for a Beta Key and try out the Windows PC game for free:

Hello Heroes,

Starting today you can register your interest in taking part in the next phase of the Battlefield Heroes Beta.

If you already received a Beta Key for the earlier Beta phase, you will be good to go. So you don’t need to sign up again. When the Beta starts, you will have access.

Sign up page: http://www.battlefield-heroes.com/news/battlefield-heroes-beta-key-signup.