League of Legends sports a hefty player base, Mac users have been noticeably absent since the previous Apple-only beta shut down in September of 2011. Associate Producer Steve Mieczkowski has announced that a new Mac version is on its way. And once it goes live, both versions of the client will receive all content, features, and bug fixes simultaneously.
The Mac client is currently playable on the game's Public Beta Environment, and it will enter an open beta phase when developers are confident with the results from the PBE test. However, players needn't wait for the open beta to try it out; in fact, Mieczkowski encourages folks to download the PBE from the official site now and help by offering feedback and finding bugs.