The iPhone has so many great music-creation apps that they deserve their own category, but shiny dedicated instruments like Korg's new 37-key sampler are always nice. It has a line-input, goose-neck microphone, and editing software for the Mac or PC.
It connects to computers via USB, but also has MIDI in/out connections. The line-in jack is what you'd use for any external audio source, including the iPhone. Away from the desk, the Korg can last about 4 hours on 6xAA batteries.
Sample modes include one-shot, gate, loop, key gate, and auto-next. What you record in is mapped onto the keys for you to play and build up layers. Sampling time/internal memory is 42.66 minutes at 24kHz quality, and half that at 48kHz.
The 21 built-in effects include all the usual stuff (distortion, EQ, reverb, delay, chorus, looper, etc), but there's no vocoder (think Daft Punk "Around the World"). Seems a little strange given the attached microphone begs for it.