florinc : yes, there is a serial port, but the official SDK won't let you access it, so you'll have to use a jailbroken iPhone / iPod to use it.
With the latest 3.0 SDK, there's at least a better chance that we can use the dock connector:
Using the new External Accessory framework, your application can now communicate with "Made for iPod" hardware accessories attached to iPhone or iPod touch through either the 30-pin dock connector or wirelessly using Bluetooth.
Not sure exactly what the whole "Made for iPod" part is about, you might have to get one of Apple's special chips or something...