Is it possible to create an app that would scan the 30 pin connector and check for some kind of hardware, say a remote controlled car, and be able to control the movements of the machine while being tethered via the usb/30pin cable?

Yes, you can!! You just need the right tools and your off. Some people have managed to create a keyboard for the iPhone working through the headphone jack and it works without jailbreaking. Here's a website link just by a single fast google search http://www.nerdkits.com/videos/rc_car/ or check out iPhone hacks a great (but outdated) book on these stuff and much more. I'm not advertising admins.