yeah , im working on something somewhat similar( though not for guitar, but more of a controller to DAW's, so i can add basic interaction to older mixers).
And that midi shield will allow me to focus on the code first and then i can start thinking on the prototype side of it. Mind you i have a quite fair good electronic knowledge and still preferred it that way. But if u on a budget, its quite easy to do one with a pot and a button to start tests.
Mind you, theres a far advanced project for DIY ( not arduino, mind) that people have done whole 48 channels studio controllers with it, and much more @ http://www.ucapps.de/. I think their newer versions use the NXP LPC1769, but they have PIC cores also.
A good example out of it is MIDIBox 64 : A DIY MIDI Controller Surface