Please see USB programming on the iPhone .
And at least you can compile libusb for iOS. I was not able to use it on an iPad without a jailbreak camera with a camera adapter, but it can work on an iPad with a jailbreak.
Associate IOKit headers with the iOS SDK on Mac OS X.
$ sudo ln -s /Developer/SDKs/MacOSX10.6.sdk/System/Library/Frameworks/IOKit.framework/Headers /Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS4.3.sdk/System/Library/Frameworks/IOKit.framework
libusb configure-iphoneos.sh .
$ ./configure-iphoneos.sh
$ make
, Iphone Android. Arduino Audio Jack .