I haven't tried it yet on my iPhone but Xtreme mac makes something called the micromemo that was like a Mic AND/OR a line input for the video iPod 5G. I used it a couple of times with that video iPod.

www.amazon.com

Hmmm, can't find anything for the iPhone. I just tried plugging it into my iPhone, doesn't see it, but I have an adapter/converter in my car that I use with my iPhone, will try that later and report back.

Interested to find out if u get it working

found this:
Watchwww.youtube.com

need a 4 pin 3.5mm (1/8 inch?) headphone jack to 3 rca (red white yellow) cable.
looks like stereo recording is not possible through the iphone headphone jack, only mono.

also, all the recording apps ive come across doesnt allow the user to set recording levels on the iphone, level must be set on the mixer. would be easy to avoid clipping at home, but might be a little bit harder at a gig when the volumes are cranked right up.

4track can record thru the headphone jack, but you need the right cable. 4track also allows you to set the levels and panning.

the mic input is indeed mono tho

catalog.belkin.com

the sound quality of the ipod is very good. but if you need something a little more robust take a look at the digital recorders like the zoom, edirol or tascam.

I'm sure you guys have found other alternatives at this point but came across this thread... SpinUtopia is releasing an iPhone app in the upcoming months that will allow you to record a live set directly to your iPhone by plugging into the headphone 1/8" jack on your phone, monitor your levels and have it record directly to your account on www.spinutopia.com and tie it to the event media for fans to search the app/site for indefinitely.

Lot of other features that will help DJ's save time, record and stream live from events and build their following!

SpinUtopia
Your Electronic Dance Music Network

A good app I found to work well for just audio recording is a app called
RODE
And for video filming with perfect audio is called
FILMIC

Note you will need a headphone adapter to use the line input
Hope this was helpful

You need TRRS adapter to record on the Iphones input. I use Voice Record Pro

RECORDED WITH IPHONE [soundcloud url="api.soundcloud.com params="auto_play=false&hide_related=false&show_comments=true&show_use
r=true&show_reposts=false&visual=true"width="100%" height="450" iframe="true" /]

soundcloud.com