Keyless Keyboard

You're familiar with fretless basses? Yesterday - under the delayed influence of having seen the Moog documentary a week ago, and the current influence of see/hearing Sigur Ros - I was thinking that somebody should invent the equivalent mode for the piano/synthesizer. A keyless keyboard. There's lots of input modes for keyed instruments that allow you continuous flexibility of pitch (e.g. pitch wheel), but it seems to me that none of them are as intuitive or flexible as simply having a "keyless" keyboard on a synthesizer would be. Given envelope modelling and such, you could even have a pretty close approximation of a piano without a fixed tuning - putting to rest the whole tempered-scale-as-foundation-of-music-based-on-western-instruments issue that I'm sure keeps you up at nights.

Does this already exist? Anyone know how to build this? I'd make you a delicious sandwich.