I really like this idea. From Manifesto Architecture.

Bike Hanger is a proposal by New York-based Manifesto Architecture to hang bikes clear off the side of buildings. A ferris wheel-esque frame latches onto to under-used facades — the back of apartment towers, underpasses, etc. — and works like a human-powered conveyor belt. Pedaling on a stationary bike nearby powers the wheel, forcing it to rotate. Cyclists then hoist their bike up on a T-shaped rack. When they’re ready to take it down, they (or a bike-rack assistant, it’s not clear who’d do the work here) would cycle away on the stationary to return their bike to the ground. In short, it’s the two-wheel equivalent of stack parking, only greener; the architects say it wouldn’t require any extra mechanical juice.