here's the simplest way to build a hidden door that is completely invisible, using 12 sticky pistons, redstone wire, and 2 repeaters.
Hook up pressure plates: http://www.youtube.com/watch?v=jdxjRgGN-D8

There are some redstone oddities that make the configuration very specific, so you need to follow the design exactly. The door in this video is facing north.

When you make your switches to this door, connect them to the roof wire, not the wires on the sides. This is because the forward pistons must retract first, otherwise the side pistons will not be able to pull them back into the walls.

The roof is also a good place to hide any complex circuits that you connect to your buttons, switches and pressure plates.