Agreed. I don't see why Apple would reject apps that connect to external sensors. There are already many such apps that use Bluetooth (eg Wakemate) or the headphone jack (eg Square), although getting approved to link up with the bottom port is supposedly a pain. If you want to take advantage of the closed nature of Apple's platform, you have to push the kind of apps they don't like. So far, that's mostly just home screen customizations and pirated games, I guess. Google Voice was looking like a killer app not-available-on-iOS but Apple backed off of that one.