Gearbox has updated Borderlands 2 for Windows. Unfortunately, the update breaks multiplayer compatibility with the Mac version, which is published and maintained by Aspyr Media.
We pinged Aspyr Media to find out what's going on, and here's their statement:
"The Borderlands 2 update released on June 26, 2013 for the PC will cause an out of sync [error] with the Mac version of the game. We are working tirelessly to ensure the Mac version is caught up with the PC with this new update, but please be aware that the Mac and PC versions will be out of sync for a very short time, disabling cross platform play. We hope to be back in sync as soon as possible. Any questions please contact us at http://support.aspyr.com and log a ticket for one of our agents!"