Here's a variation on that Virtual Keyboard that's been floating around for a few years: the Virtual Piano from Digital Information Development (DID) uses a highly portable red semiconductor laser module to project a keyboard onto any surface. The device then automagically detects the position of your hands and plays the resulting music through an onboard speaker, but if it's anything like the Virtual Keyboard, it's flaky.
There is good reason why the Virtual Keyboard took so long to get to market, and when it finally did, it was not the huge hit its developers had hoped. There's no touch sensitivity and it feels just like what it is: typing on a table. This is an even bigger drawback when you're talking about playing music. We're thinking this Virtual Piano is another one of those devices that's much more impressive as demoware than as an actual product.