Hearthstone, told Joystiq at PAX Prime that an expansion for the game will be coming "soon." The expansion will add new cards and lots of them - more than 100, actually.
Unfortunately for those who enjoyed taking on the hordes of the undead in Curse of Naxxramas, the expansion will not feature additional single-player content. Dodds clarified that Naxxramas is an "adventure," while what's coming is an "expansion."
Still, don't rule out more adventures in the future; Dodds said that, while Hearthstone is "primarily a PvP game," the single-player content allows the game's designers to steer players into trying new classes and strategies. The goal is that players will take those strategies into the multiplayer arena, but even if they don't, it provides an avenue where players can think differently than they're used to.
As for those iPhone and Android versions? "Still in progress," Dodds told Joystiq. But they should be *ahem* ready to ride by the end of the year.