I have installed a plugin that allows you to build retractable bridges. Here is a link to the information about it at Bukkit.org: http://dev.bukkit.org/server-mods/retractablebridge
I have changed the plugin a little to allow us to use full size blocks, wood, stone, glass, iron, netherbrick, and gold blocks all work. There are a couple of sample machines over in the Redstone Testing Center in the Creative world.
If you run into problems with a redstone machine breaking part of your house let me know. Basically try not to run redstone wires 3 wide under unconstrained blocks above them. Let me know if you have any issues and I can change the plugin.
Retractable bridge Plugin
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