
Key points:
- Overall the functionality would mimic Pioneer CDJs + DJM mixers.
- Eject button would allow the user to select a new track from their music collection, bringing up a new screen to do so.
- Knobs would be activated by the user pressing down on them and holding as they slide their finger up/down or left/right, and deactivated when the user released their finger from the screen.
- Active knobs/faders/pads would light up/glow to indicate they are selected. No other knobs on that channel would be affected if the uses finger slid over them.
- Double-tap would return knobs to their default position.
- Faders would be controlled with up/down finger movements in the same manner as knobs. Thanks to the multitouch capability of the iPhone you could adjust EQs + faders on both channels simultaneously.
- The time info display above the play/cue buttons would also serve as a nudge pad, where the user could tap + slide up/down to nudge the track. This should probably be made clearer in the UI.
- Sound output through the headphone jack would be split: Master on the left, headphones on the right. This means all output would be mono, which sucks, but it's better than nothing. The user would buy a stereo -> 2 x mono cable splitter like this:

- It might also be possible to output the headphone output via the stereo jack and master output over wifi, but if this incurred a delay it might make DJing difficult.
The only thing I can think that's missing is effects, booth monitor control and beat syncing. I'd argue that effects aren't high priority enough to take up screen real estate, beat syncing is for pussies, and the booth monitor could just be the master output signal split again. This means your booth speakers would need a hardware volume knob though.
Obviously this wouldn't be a replacement for decks, but it would be great fun for house parties - certainly more useful than a lot of the current "DJ" apps on the iPhone. My biggest concern is that the screen is simply too small to fit the number of controls on at once. What do people think?