I've just finished a project I started months ago. Here's a working layout for the IK iRig version 1 for iPhone or iPad. I am going to use my iphone as a strobe tuner for guitar and bass setups, and for recording demo songs on trips and such. The main advantage of me building this rather than buying it, is my construction is a lot more robust than the plastic IK unit, and I used switchcraft jacks, star quad cable, etc. Between this, and my two Tascam iPhone interfaces (line input, and stereo XY mic) I'm covered for portable audio. Amazing.
In use, with IK's free Fender and Marshall Amplitube apps, there is noticeable latency when playing guitar, but it's tolerable. Also, this iRig circuit is pretty noisy, and the output is pretty low. Thankfully IK includes a noise filter as one of the selectable effects in the Marshall app. The sound quality of Amplitube is harsh and gritty, but, it'll do fine for jamming, writing, or demo recording. I would not use it for critical recording.
Here is some technical information:
And here's the schematic, my stripboard layout, and my finished unit.