Because someone had to do it...
The MFi Program
The arrival of the External Accessory Framework was seen by many as having the potential of opening the iOS platform up to a host of external accessories and additional sensors. Sadly little of the innovation people were expecting actually occurred, and this is usually laid at the feet of Apple's Made for iPod (MFi) licensing program. The restrictions placed on developers by the existence of the MFi program have meant that connecting your iOS device to an arbitrary piece of existing hardware was hard.
The Redpark Cable
However many of those pieces of existing hardware have serial (RS-232) interfaces, the arrival of the Redpark Serial Cable makes connecting to such hardware fairly easy.
"iOS Sensor Apps with Arduino," by Alasdair Allan (O'Reilly), provides a solid introduction to the topic of how to connect your iOS device to the real world. It guides you hands-on through connecting your iPhone and iPad to serial devices using the Arduino as an example platform.
By the end of the book you'll be able to use your phone to talk to serial hardware, build a sensor network, and make your iPhone part of the Internet of Things.