Seeking recommendations for a spaced-repetition system that syncs between your iPhone and your desktop

I recently got an iPhone with one of my main reasons for doing so being to productively fill the time I have on the Tokyo subways when I can't grab a seat and break out my laptop. One of the things I intend to do with that time is using a spaced-repetition system ("SRS") to help expand my Japanese vocabulary. So I'd like to see if anyone out there has any recommendations for such a system.

There are a few features in particular that I'm looking for.

Read more... The most important is the ability to sync between, on the one hand, a desktop SRS app, such as Anki, Mnemosyne, or SuperMemo and, on the other, whatever I use on the iPhone. The idea here is that if I use the desktop app for a minute while on hold on a call at the office and then study on the iPhone on the way home from work, when I get home and sync with my desktop app I'll have my most up-to-date learning data. Accordingly, simply importing from a desktop app is insufficient for my needs.

The second feature I need is that I have to be able to use it on the iPhone while not connected to the internet. Part of my commute to work is underground (and it's also the part of my commute where I'm most likely to not get a seat), so needing an internet connection will not work. Accordingly, I can't simply use one of the SRS websites out there.

Finally, it's gotta be easy to enter the things I want to learn. And that means that I won't have to do it on my iPhone.

So... any recommendations?

I've so far only scratched the surface in my own research (and I'll of course report back when I have more), but I've been considering using Anki's iPhone system, which isn't an App Store app but somehow works nonetheless. I've also been trying out StudyArcade (and considering its $4.99 pro version), but I've yet to fully explore how it syncs up with Anki.