Anki decks can be exported into the lowest-common-denominator format among SRS apps: tab-delimited text files. They're supported by pretty much every app worth using, and if they aren't, easy to transform them into something the app does understand.

From the discussions on the Mnemosyne-proj-user mailing list which I didn't pay any attention to, there are apparently a number of free SRS apps for the iPhone.

(What isn't usually all that portable is the markup for formatting, images, audio, etc. But I don't think any of these cards are likely to involve much of that.)