For full info: http://forums.terraria.org/index.php?threads/terraria-1-3-0-7-mac-linux-open-beta.27418/
And to report bugs or provide feedback: http://forums.terraria.org/index.php?social-forums/official-terraria-mac-linux-open-beta-testing-feedback-group.690/
This is nice to hear! I can't wait to try this out tomorrow.
Updated!
Tried the Mac beta and it worked great. I can see why it's still beta but it's a great first start.