I used to think that very same thing as well. I also used to think that about my Xbox until I realized there was a whole other world out there. More of an issue of 'whynot' instead of 'why'. I have seen discussions and screenshots of people using their 3 and 4G's as web servers and such. If someone is going to turn their's into a friggen web server then a recorder doesn't seem to be far off or implausable.
As far as a company making a new dock connector, i'm sure all it will take is some college kid majoring in elec. engineering to come up with something. Hell maybe even leegeddy can conjour up something.
The XBox analogy is way off. What did it take to turn the XBox into a webserver? Simply a modchip, which is made to play pirated games anyway. There is a much larger market for modchips than webservers, which is why they can afford to be made.
Dock connector inputs, however, are very specific to our hobby. There are already recorders available for the people who want to use the iPod for lectures and such, so there would be little to no market there. The taper market is so small that it makes very little sense to create a device specifically for us. If you look at the rest of the devices we use, they're all professional devices that can be used outside of our hobby. Even the 722/744 are marketed more towards film professionals than us.
I think you also vastly overestimate the necessary hardware required to create such an input. If it were easy it would probably be done already.
But again, I could be way off base. All I know is I've been hearing questions about iPod recording since 3 or 4 years ago and not once has anything come out of it.