At last! Good man!
Your mockup looks pretty decent as it is..
I wonder if it might be a sensible idea to have the main menu options locked and thus directly accessible from any point in the system? As it currently stands, theatre view takes an awful lot of click throughs to get to where you want and then even more to get back... Also, be sure to make the currently playing track information visible from every page - not just 'playing now'!!
How about having a section of the screen showing some playlist stuff - name of playlist, playing now, played last, playing next? Sod the visualisations - if you want to see them then skip to full screen view where they can be properly appreciated.
No-one has made a brushed steel theatre vew interface yet either (just my preference). They always look suitably 'plimp' though...
The one thing that theatre view could absolutely do with though - movement! And by that I don't mean some half baked visualisation cooking away in the corner but proper menu transitions, icon rollovers etc. Flash maybe? Look to MCE for inspiration i guess - despite it's many faults, for UI flashiness, it is certainly impressive. Sort that out and most of my issues with theatre view will be solved in one fell swoop.
In track info, just the cover art, artist, title, album - maybe the year? if it uses a track info template then users could just edit their own preference I suppose. An automatic link to some artist info (from allmusic or something) would be pretty cool though - would become a bit of a talking point at parties I reckon. Winamp did something similar I seem to recall...
What else? Sorry - this seems to be very 'functionality' based, which I'm sure you didn't actually ask for.. How about incorporating a means of accessing internet radio?
Anyway, can't wat to see what you come up with,
Mr Morkey.