radio, etc)? Similar to a traditional front-end like RR or CF. I might be wrong, but once you have the app store version, it shouldn't be too hard to add the things that apple doesn't allow to make it more like a traditional front-end.
Also, you were talking about using a backend that the ipad could interface with. This might be a good choice: http://www.engadget.com/2011/05/02/c...er-users-only/. A little more pricey than the sheevaplug, but it's smaller, more powerful, and has more I/Os, including an RS-232, and with windows releasing and ARM version soon, you I'm sure drivers would be less of an issue for hardware.