Is there a guide or documentation for designing and interfacing with iPhone4 hardware? How can I design peripheral devices that plug and communicate with iPhone4 and its iOS4, so that I create apps to use these peripherals? What accesories should I use to do that?
I am already used to designing and interfacing using serial and parallel on memory-mapped devices (e.g. NIOSII on Altera DE2 board with any device).
To get official technical information about the hardware, you have to join Apple's MFi program. See http://developer.apple.com/programs/mfi/
Note that you'll have to sign an NDA to join the program. That's why you don't find a lot of this info floating around on the net. However, if you search hard enough, you can find people who have reverse-engineered some aspects of the hardware. For example: http://boingboing.net/2010/08/04/reverse-engineering-2.html