With electronics ever-improving, the portability of what could be called a musical instrument will only increase. I saw someone on the train playing the drums with his thumbs on his iPod Touch. We can play Guitar Hero on our Nintendo DS consoles and if you use the right software, any laptop is a sythesiser. I also glimpsed an ad for a "Paper Jamz" electronic toy guitar that I guess is comparable in weight and thickness to cardboard.
So sometime in the not-too-distant future, I expect it will be possible to carry a guitar, piano or almost anything just about anywhere, folded up small enough to fit into a pocket, and wirelessly connected to a media player for sound output. It won't be exactly like traditional instruments, but that is never the point with new technology.
Mokalus of Borg
PS - Now if only I knew how to play anything.
PPS - None of my self-teaching experiments ever got far.