There are so many applications that could benefit from hardware interfacing to a smart phone that it's hard to make a list. Apple makes doing this very cumbersome and expensive. Just to be able to talk to the serial port you have to be approved for MFi membership and comply with a million requirements that only add expense to a product. That's why you don't see a simple serial cable available for open use. You can't legally buy the connectors and you have to also buy a decryption chip even to use the simplest functions.

That's why you have people resorting to the ridiculous (but it works) extreme of using the headphone jack to build such things as credit card scanners and the like. They don't regulate the use of the headphone jack and seem to be approving apps that use it. Headphone jacks were being used way back when (Commodore 64 and TRS80 days) to talk to cassette-tape based storage. It's literally going back 30+ years in technology.

While, on the surface, open peripheral port access might seem like a geek feature I think that this is far from reality. If peripheral port access on the iPhone was open for all to use you'd see all kinds of very interesting things happening with hardware for the iDevices that you simply aren't going to see given the current framework.

Also, this is the reason why there are no inexpensive iDevice compatible hardware products. Developing them is very expensive due to the nature of the process.

If MS was open about their I/O and they achieved reasonable penetration there's a whole layer of the product world that would open up to their tech.

I think that when you come from behind and have the financial resources you just can't afford to ignore any market, no matter how insignificant it might look from your current vantage point. You never know where the next killer app might come from.