The guitar adapter thingy from Amplitube.
It's the iRig adapter for iPad & iPhone.
It plugs into the headphone socket, with a pair of sockets on the unit to accommodate your headphones (or line out) and a guitar input.
Unfortunately the iRig doesn't work very well with Apple's Garage Band v1.3 Tested on an iPhone 5 (iOS 6.0.1)& a 3rd Generation iPad (iOS 5.1) and it happens on both units.
|Garage Band Versions|
This is a real bummer, because Garage Band is a really nice bit of software.
Rumour has it, that the previous version of the Garage Band software worked just fine.
Hopefully it'll be fixed soon, otherwise it'll just be another FX toy that I don't use much.
I've managed to downgrade Garage Band from 1.3 to 1.2.1 on my two iOS devices and the feedback problem has been cured. There is the issue of a "ticking" noise on the iPad, but I don't have this problem on the iPhone 5.