Hearthstone has amassed a huge following of over 20 million players. Currently available for PC and iPad, Hearthstone has been earlier announced to arrive on Android and iPhone but as a blog post in the Hearthstone blog says, the devs are planning to release the mobile versions of the game early 2015. That could easily be Q1 or early Q2.
Hearthstone for Android and iPhone has long been in the pipe and the blog post states that the devs are taking their time to fully guarantee a smooth exprience for mobile devices.
Read more about Hearthstone in the official Battle.net page from Blizzard here.