siBlizzard has confirmed that both versions of Hearthstone, for iPhone and Android, will support cross-play with the PC, iPad, Mac and Android tablets versions. One of Blizzard’s designers who worked at Hearthstone talked to Gamespot and he said that when you’ll play you’ll be able to know from which device your rival is playing:
When you start a match on a phone, you could be playing someone on another phone, someone on a tablet, on a PC, who knows? It’s all one big ecosystem.
The version of Hearthstone for smartphones was firstly suggested last month, and probably it will come out in a few months. However, until then, the developer is working at the next adventure, Blackrock Mountain, which will be released next month.