Spotify has updated their mobile app for the Apple iOS platform. The updated edition feature support for 320kbps song quality.
This can be selected from stream settings which offer three options: Low/High/Extreme. Extreme would stream/cache 320kbps quality songs on your phone.
It is not clear if Spotify would make this feature accessible on their other mobile apps but we see no reasons why not.
Extreme quality is available only to premium customers and you would want to be careful about your bandwidth bill if you take this route.