The bad news for Mac users who may have been looking forward to playing the game on their computers, is that Riot has decided to close down their Mac operations indefinitely (which never sounds good).
Their reason is because they were unable to “guarantee the quality and frequent updates we deliver today on the PC and are expected by our player community”. However they did state that in the long term they have plans to make the game playable on a Mac, but as to when – that remains to be seen. Of course there’s always the option of installing Boot Camp and getting a copy of Windows should you have the strong urge to play the game.