MacRumors detailled DJay app last week which took advantage of a number of new features of 4.2:
- Full access to iPod library
- Multi-tasking: you can run djay in Automix mode and listen to a continuous, seamless mix running in the background while you surf the web, play games, etc.
- AirPlay: you can wirelessly stream your mix to your Apple TV or AirPort Express station in real-time.
- Fully leverages accelerated CPU extensions (SSE-like) for high-quality audio processing and analysis
- Very low latency (< 3 msec)
- Background audio playback (multi-tasking support)
- Pre-Cueing (via mono/stereo adapter)
The first crop of apps that use CoreMIDI on 4.2 with the USB Camera Connection kit are hitting the App Store.