Draper Micro Projector Lift ($2,600): The Draper projector lift uses three scissor lifts to stabilize the projector while a tubular motor lowers the unit down as much as 48" with a 35 pound capacity. The lift is expensive, but Draper will factory install cables for RGBHV with BNC or 15pin HD connectors, S-Video, Composite and Component Video with BNC or RCA connectors, Serial Communication for projector control with DB9 connectors, CAT 5e, CAT 6, and HDMI.
Focupix Electric Projector Lift ($1,000): Just under a grand, the Focupix lift is somewhat of a bargain. This model uses three scissor lifts to drop down as much as 35" from the ceiling and works with both RF and IR, offering flexibility in integrating into your automated home theater setup. The Focupix supports up to 50 pounds and installs into 2'x2' or 2'x4' suspended ceilings.
Draper Phantom Projector Mount ($2,200): If you don't have the vertical space in your ceiling joists for a scissor lift, this model takes up a little less space by tilting down instead of lifting up.
Draper SLX17 Projector Lift ($6,800): This scissor lift is absolutely insane, capable of lowering up to 350 pounds to a distance of 17'. If you've got a heavy duty high end projector, extremely high ceilings, and a big budget to match, this puppy is your weapon of choice.
DIY: All very expensive options, but if you've got the DIY wherewithal you can try your handing making your own!