Flatwire, makers of specialized cabling that can be hidden in drywall with a minimum of extra work, have announced a new HDMI variant in lengths up to 20 feet. There's no price yet, but if it's anything like the component video versions, it won't be cheap. I would expect a 20-foot length to be around $140 or more.
Far more expensive than your generic HDMI cable from Monoprice, for instance, but not all that much for a nice media closet installation. Should be out in the new year.