I know Sencha Touch 2 is supported only for iOS 4+, but in case any legacy 3.x installs my app, what functionality/controls could have issues? (so I can maybe avoid using some of them?)
In a perfect world it would be great to set the minimum iOS version to allow the app to be installed to but AFAIK even Phonegap doesn't have this ability.
You could check the version and display a notice that you must have iOS 4+ in order to use the app.