Are the instruments in GarageBand for iPad just not good enough for you? Amidio is in the process of making a universal iPad/iPhone case with Bluetooth connectivity and... wait for it... a dedicated ukele synthesizer app. Futulele will support 12 chords for each song, with the ability to switch chord sets easily. Sharing, recording, and effects from their previous app, OMGuitar, will be ported over to the app.
Of course, we've seen accessories like this before, such as Guitar Apprentice, which is built to be more of a faithful reproduction of the real instrument, and honestly, this seems like overkill give that uUkeleles are really inexpensive -- you can get one for $30 or so. If iPad connectivity is really that important for sharing or editing, why not just use it to record your performance on a real instrument?
Futulele is still in the prototype stages, and Amidio is hoping a manufacturing partner will be able to squeeze in some dedicated speakers into the case as well. Anyone interested in this sort of thing?