Secret Base Designer Fessaboy immediately snapped up the JACK Audio Connection Kit SDK yesterday and started adding support in their apps! Jesse from Sonosaurus also announced that Thumbjam would be updated with support for JACK.
Fessaboy was commenting here yesterday with some excellent developer insights. He brought up an excellent point I had overlooked when I welcomed JACK as our new VirtualMIDI Overlords. For controller apps and sequencers that currently use VirtualMIDI to send MIDI to multiple apps, JACK is kind of a step backwards as you have to go into JACK to configure those destinations. For Secret Base Design's own MIDI Control this doesn't make a lot of sense, since you can just select those destinations right there inside of MIDI Control. He had good things to say about using JACK for Audio and the ease of implementation, but doesn't feel it is going to replace Audiobus.
It is now a race to see who gets to be the first third-party developer with a JACK app on iTunes, but it looks like Secret Base Design is in the lead with this demo of Live Guitar.
Amusingly this video was tweeted out with the chords for AC/DC's The Jack, which can be imported into Live Guitar. So meta!