I think the key here is the cable connection - if it's plugged in, it assumes the device is an ipod and automatically defaults to iTunes.
If you just connect via bluetooth, it works as desired - just mirroring the sound from the required app on the phone. Spotify also passes track and album info, and you can navigate the tracks in any album / playlist using the forward and back keys. There is no album art via bluetooth for any app (I think it's not part of the bluetooth standard, but I may be wrong).
It is frustrating though, because Spotify also passes album art via the physical connection, but when you change tracks, it defaults to iTunes
