If you're considering creating a GUI using a javascript library anyway, why don't you just write a web app instead of an iPhone-native one? It seems like overkill to jailbreak the device just so that you can install a ruby + sinatra web app on it. Can't you just put the ruby web app on a server and create a Safari shortcut to it on the home screen? If you don't have a server, you could always run the website off a PC in your home...
Jailbreaking the OS and running a ruby app onto it would be technically cool, don't get me wrong - I just think it'd end up being a time sink.
Just my 2c!
