This is probably ancient news for the biggest of big fans of Catan [$4.99 / HD] in general, but check this: USM, the same dudes responsible for the App Store port of the popular board game, are putting out a port of Rivals of Catan. This morning, actually, we were told that the "first version" is currently in testing, and USM is inviting its users into a beta to give it a spin.
Rivals of Catan is a card game adaptation of Catan proper built for two players. In brief, it's all about expanding principalities and scoring points for various things in a fairly rapid-fire fashion -- players roll dice for bonuses and draw cards each round, some of which activate a special event or allow general improvements. You can read more about it here, if this is tickling you in any special way.
And by the way, that Cities & Knights expansion is still coming to Catan. A beta test is planned to start at the beginning of the year, and DSM hopes to release at some point around February or even March. Cross-platform multiplayer is also still scheduled to hit at some point after this update, FYI.