I'm a big fan of Ascension, the iOS app that replicates the real-life deck-building card game of the same name. But when it comes to more self-contained deck-building card games where you build a deck as you play, as opposed to collectible card games like Magic: The Gathering, there's really one big name out there, and it's Donald X. Vaccarino's game Dominion. It's a great game that like Magic: The Gathering has been begging for a solid iPad/iOS port ever since Apple's platform first took off.
That hope is coming true in more ways than one. There's already a version of Dominion available on iOS for US$1.99, but apparently it's unofficial yet (temporarily) approved. It may be strange to see a paid app that's unofficial and actually uses the art and IP of the game, but apparently Rio Grande is cool with that, because it has an official version coming out as soon as "a few weeks" from now. It's granted temporary licenses to developers to release their own versions of the game as long as those versions are down and gone by the time the official release arrives.
Personally I'll probably wait for the official version, especially since it sounds like the wait isn't that long. But it's good to hear that one of the best card games around is coming in virtual form to Apple's tablet. As for Magic: The Gathering? Wizards of the Coast, the ball's in your court. Wizards has said it's releasing a reference app for Magic on iOS, but there are still no plans for an official version of the game itself. Meanwhile, Kard Combat is it.