iPhone Adapter 3.5mm male to 3.5mm Headphone Jack and 1/4in Microphone Jack -
They have other cables made specifically for the iPhone which may be better suited to what you want to do but this cable has allowed me to plug my guitar or mic directly into my iPhone with the 1/4in input jack. I promise you, this is the cable you’ve been looking for! If you are recording lectures, pod casts or speeches with your iPhone, they have a cable with an XLR hook-up so you can go directly to a high quality, professional microphone. Although, if you want to use a condenser microphone you will need phantom power from a compact mixing board or preamp. You can also use the mixing board to record multiple instruments are microphones at the same time.
Behringer Xenyx 502 Mixer - Available from Musiciansfriend.com
The compact mixing board can also offer a solution to the limitations of apps like FourTrack like monitoring what you are playing while listening to what is already recorded. You can plug your headphones into the compact mixing board and then plug the microphone output on the KVC cable into one of the channels on the board. You won't be hearing what the iPhone is recording, so you can't tell if it's clipping, but you can hear your audio playback and what you are currently playing at the same time.
Audio-Technica AT2041SP Microphone Pack - Available from Musiciansfriend.com
I use the Audio-Technica Microphone pack for recording acoustic guitar and vocals. If I'm recording a live session in the studio, like a band practice, I will use two of the AT2020's as room mics. MXL also makes a cheaper microphone pack that is more affordable and has decent sound quality. You can use pretty much any microphone you want with this set-up but if you want to use a condenser microphone you have to run it through the mixing board or a preamp with phantom power.