IMO this is the highest priority issue for Zcash right now. Until it happens, Zcash on mobile is just BCH or LTC or something like that.
Currently you need a full node to do a shielded transaction. What about some kind of light client/wallet on mobile that facilitates a secure connection to your home server/node that does the actual transaction? The only other alternative is to have a full node on your phone, right? Something which is not necessarily crazy considering how much storage phones have these days wrt to the current blockchain size.
Then there’s the political side too… if we want it on iOS, that requires Apple’s blessing. Anyone know if discussions about that have been going on?