There are a number of companies offering products that let you plug your guitar into your iPhone. Most of them assume (astutely, perhaps) that musicians don't have a lot of money to throw around. Products like Amplitube iRig or Griffin GuitarConnect offer ways to transform your iPhone or iPod Touch into a virtual practice amp for around $30-$40. The result is fun and arguably convenient, but the sound quality isn't great.
Sonoma Wire Works takes a different approach, with a $199 accessory called GuitarJack that works directly through your iPhone or iPod's dock connection. The product isn't cheap, but neither is the sound, which can be adjusted for 60dB of continuous analog level control, and offers simultaneous recording from both instrument and mini jack inputs. Each input can also be adjusted with various gain pad options, and switched between hi-z and low-z impedance to precisely account for instrument and line-level sources.
The design and build quality is first class. The adapter is housed in an aluminum casing that feels as if it could survive a nuclear bomb. Even the input connectors use a high-quality nickel-plated brass that can hold up to the best pro-audio gear.
To put the GuitarJack through its paces, I tried it out yesterday morning using my guitar and the FourTrack app. To see (and hear) the results, check out the following gallery.