I've been collaborating with the guys at PRS guitars and http://www.bondmusicresearch.com to produce an iPhone app for guitar noodlers, and it's in the iTunes store today!
To view it in iTunes, click here:
The basic premise is that it lets you loop and time-stretch sections of songs so that you can practice along with them - we came up with a pretty neat scrolling/zooming waveform view for that. PRS are also releasing a cable that lets you plug your guitar into your phone, and if you've got one of those, the app provides some basic guitar effects for playing it live (we'll probably add some more serious amp simulations in the next version).
The app doesn't actually use Juce, but the process of writing it was my way of learning the iPhone API, and I developed the Juce iPhone port at the same time. So as well as being a cool app, it was a useful learning experience!