There are several implementations of OpenPGP on Android and IOS. These guys have one that's coming along nicely and has OK email and XMPP integration (because they actually wrote a decent API for (de)crypting).
Of course, your keys are only as secure as your phone... which isn't very. Google, the carrier (for stock, branded phones), and who knows else can remotely swipe the key from your device. There is rudimentary support for secure elements like YubiKey, though.