I must be missing something because I don't get using an iPad or an iPhone at a restaurant as being "rude" by definition.
These and other electronics are all devices that can be used and abused in many fashions. If you recall the Nextel Direct Connect, that was very similar. I hated it myself because there was no "leave me out of this" option if you were near someone using it. That's fine for on the job, like at construction sites or doing tech work on a campus that I interviewed at. But in a restaurant, turn the damn thing off.
But iPads and iPhones can be very useful in a restaurant. First, if you go to eat by yourself, you've got something to do. iPad can be extremely useful as a book reader in this instance. I'm assuming at some places you can even get work done, especially if you have a keyboard.
Move to what the OP suggested. I have thought that at least the menu part is an awesome idea just waiting to happen. We already have apps and websites to place orders at pizza delivery places. Starbucks shows you what kinds of drinks can be made. I don't know of any that do nutrition, but Weight Watchers has an app that does that, and others have similar apps.
I don't know that the actual ordering thing is necessary since it is nice to have an employee to whom you can ask questions. But I would personally love a menu app that would explain all sorts of stuff instead of pre-printed menus that have to last for months and can't give you specials or stuff like that. And many places have alcohol listed but don't give you prices because that stuff changes. With an app, that's gone.
Now I understand that it can be rude to just use something like this and ignore everybody else if you're in a group. But I'm usually with my parents and many times with my young nephews. I can keep my nephews entertained with the Crayola ColorStudio HD and SoundTouch apps. All of them love to see pictures, which I can show them on the big iPad screen. A time or two I have shown them videos I made using the iMovie trailer feature.
So yes, these things can be abused in restaurants. But they can also be used quite well.