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:
http://www.danacsimmons.com/content/working-iphone-audio-input
And here's the schematic, my stripboard layout, and my finished unit.
UPDATE - REBUILD ON 5-23-20
I was having some problems with my phone and my old build. I decided rather than to troubleshoot I would rebuild in a new enclosure. I re-used the circuit, though. New TRRS cable, new enclosure.
It didn't work right away. The two problems I found (d'oh!) were not sanding the paint off where the audio jacks touch the enclosure for grounding, and also had to spray some contact cleaner into the headphone/mic jack of the iPhone. Once I did those two things it now works perfectly, very reliable, no weird cut outs, noise, etc.
Really obvious stuff. Make sure your grounds are actually grounds. Make sure your connections are all clean and reliable. Had to sleep on it, but solved it quickly the next morning.
Also, some people are using the J201, or the 2N5457 in place of the BC245C. You would have to mind your pinout (drain source and gate pins). There are reports of different gain levels with these substitutions. I haven't tested them. They might be easier to find than the BC245C.
Also noticed my iphone is putting out about 0.88 V to the FET, not 2.7 V. But it still works fine.
Here's the new re-build:
