A free Ear Trainer has just arrived on iTunes!
This app will aid in developing a familiarity with the sounds of specific intervals and their physical separation on the fretboard in various scales. The focus of this app is on the seven modes of the major scale and their tonal flavors. Play with each scale to get used to the intervals involved and then test yourself by hitting the "play" button, which will begin the game. With practice, intervals can become distinct and recognizable sounds. Then, all of music can becomes more accessible and yet deeper in meaning.
It calls itself a "Beta version" in the app, so it may be going up in price when it is final.