Is the proof-of-work likely to be too much for a mobile to be able to do? If a mobile client were to be made, that is. Also, does the proof-of-work scale, eg h

Is the proof-of-work likely to be too much for a mobile to be able to do? If a mobile client were to be made, that is. Also, does the proof-of-work scale, eg how the bitcoin network gets harder and harder as the hashing power increases.