At the moment we're focusing on iPhone, mainly because it makes things a lot easier to target such a consistent platform - if there's only one hardware maker then there's only one screen size / processor speed / camera resolution (if we ever get around to introducing a camera OCR system) / etc to deal with. Plus, with at least three different OSes competing to be #2 behind iPad in tablets, we think Apple is poised to continue dominating in that market at least for many years to come, and that's probably as big a part of Pleco's future as smartphones at this point - maybe even bigger if they catch on with schools.
Android is *incredibly* hard to port a product like Pleco to - too many different OS versions / hardware configurations, not to mention the need to rewrite everything in Java - and Windows Phone 7 hasn't proven itself in the market at all yet, so until the Android development situation stabilizes or Microsoft demonstrates consumers are actually buying WP7 we'll probably keep our focus on iPhone; the most likely second platform at this point is actually desktop Windows since that's an easy port from Windows Mobile.