Based on its experiences with the beta, AppleInsider is reporting that iOS 4.2 supports the CoreMIDI framework (this also appears in OS X), which suggests that developers will soon be able to develop MIDI-capable apps.
Line 6 recently took a step towards more open iOS MIDI support by making its MIDI Mobilizer SDK available for free - thus opening up the possibility of other apps supporting its MIDI Mobilizer interface - but if the CoreMIDI implementation works as you might logically expect, it should mean that developers will be able to easily add MIDI support as standard.
What remains to be seen is whether you'll simply be able to plug in your USB MIDI controller keyboard - via the Camera Connection kit on the iPad, for example - and then see it working plug 'n' play with MIDI-enabled apps. We've known for a while that this works for some audio interfaces. Furthermore, does this mean that interfaces such as the MIDI Mobilizer will 'just work'?
This development throws up other questions, too - will MIDI work over Wi-Fi, for example - but it's assuredly a good thing. iOS 4.2 is due for release in November.