Line 6 Sonic Port

If you're looking for a simple, guitar-focused interface for your iThingy, then the Sonic Port could be right for you.

Looking somewhat like a miniaturised high speed train (but less likely to cover you with hot soup), it feels very solidly built and is a great size to take with you when travelling.  One end has the interface cable connector (comes with both lightning and 30-pin cables) and on the other, 1/4 inch guitar in and mono/stereo line out, 1/8 inch headphone and stereo line input.

The full version of Mobile POD is unlocked as a result of hooking up the Sonic Port, which for my money is actually better sounding than Positive Grid's JamUp Pro (although both are surprisingly good).  Works with any other CoreAudio applications too (e.g. Garage Band).  Sound quality is excellent, and it is fully powered by the iDog.