Most of you may already know this but with the (not-so) recent influx of mobile music apps (some of which are actually TRULY awesome) this might be of some use to those who don't.

You can connect any Audio Interface or MIDI Instrument to your iOS device as long as it is Class Compliant, meaning, it does not require extra drivers, it uses those which are built into the host. This works best with the Camera Connection Kit 3 because otherwise, the iOS device will need to power these devices on it's own and this can be an issue, initially and over time. At this point, your iOS device will see all the I/O of your interface and be able to use them and it's processing power for DSP. God willing, you will now be able to run your favorite app(s) at 96 kHz with a buffer of 64 samples and near-zero latency and use all it's ins and outs.

Here's the awesomely cool part:

You can connect a USB-Hub (preferably self-powered) to the CCK3 and then connect all your devices to that.
Now you have your interface and, say, a BCF2000, a BCR2000 and an MPK249 all connected to your device. The best part about this is that you don't have to use Network Sessions over WLAN, your device will just see them as long as the app itself is MIDI capable and there is, from my experience, 0 latency and 0 dropouts.
Not to sound like a fanboy because I am absolutely not but I would recommend an iPad as it has greater compatibility with more hardware devices. Plus, all that hardware surrounding a tiny iPhone screen would look a little funny to anyone walking by.
Not that we should ever care.

In conjunction with apps like AUM, AudioBus and modstep, this is a true mobile working solution.
Of course, a laptop, IMHO, is still a better one. This can be a little simpler, though and sometimes simplicity can be key.