This is really a tough question for us. Adding iOS and Android apps might improve our visibility among Chinese students, but would also mean putting a lot of effort into developing separate interfaces for devices using entirely different development toolkits. This would come at the expense of rolling out more general features or branching into other languages like Spanish which might also broaden our visibility.
Since it is possible to listen to our MP3 files on both Android and iOS phones using existing software, we haven't committed to developing standalone applications for that. Instead, to the extent we have a mobile strategy, it is to develop mobile tools that serve more limited functions and then improve them and integrate them with the site based on feedback and our own feelings. This is what is happening with our iOS Writing Pad now, which is getting a facelift and the introduction of higher-resolution vector graphics. We may add extra review tools to the same app so that it becomes a more general review tool. What this means practically is that our Study Center is more likely to "go mobile" than our lesson archive.
As far as Android -- it's easier for me personally to prototype stuff on iOS since I'm familiar with the development environment. A decent number of people have written-in asking for Android support, so it is something that we know has to come eventually. That said, I'm also not sure it makes sense to explicitly port anything to Android until we have something we're happy with that works with iOS.