Some friends who share an apartment and both bike to work everyday tell us that they've solved it once and for all with a Minoura Bike Tower. They house two bikes, but the spring loaded, floor to ceiling aluminium alloy system can actually accommodate up to four with the addition of extra hooks.
Our friends say: "It's awesome. Keeps bikes off the ground and doesn't require drilling anywhere. Fits great in apartments with plaster walls and ceilings."
The Minoura is on sale right now for $99.95 at Bike Nashbar.