What is the most efficient village housing layout?

Here's the best solution I could come up with:

Some details:

• The bottom floor is one block off the ground; in hard mode this prevents zombies from breaking down the doors.
• The buildings are 7x7 with a 1 block ledge on the higher levels, so each has a 9x9 footprint. (The footprint expands to 11x11 when you consider the mandatory airspace that must exist about the complex for the village code to register the door properly.)
• There are a series of staircases inside the buildings to access the higher levels.
• I built columns between doors because I found in practice the villagers had a hard time navigating through doors touching each other.
• Each floor is 12 doors, yielding > 4 villagers per floor
• There's a 3-block gap between buildings which gives a 1-block gap between ledges. This ensures the 'outside' is computed properly by the village code.
• On higher levels I have connecting blocks between ledges at the intersection of buildings; these do not interfere with the 'outside' computations, as they aren't directly in line with any doors.

Now that I have plenty of villagers it's a matter of pruning out the bad ones.

This is what I've been using:

Basically the floor plan is like this:

#DDDDDDD#
#DDDDDDD#
#DDDDDDD#
#DDDDDDD#

With a ledge and stairs for easy access. I am reasonably confident that all of the doors are registering for the villager spawning deal. I haven't razed the rest of the village and counted yet though.

Just going off of the spawning rules on the wiki, I have a 9 doors per 108m^3 (6m by 6m by 3m) design.

• O = Outside block. The three blocks for this are air blocks, and all blocks above must also be air blocks.
• U = Unused/Doors. I'm not sure whether placing doors here will be more efficient. For aesthetics, I'd say keeping these as a solid block would be useful.
• D = Door. The block above should be a solid block.

• I = Inside. Two air blocks with a solid block above.

  OOOOOO
  OUDDDU
  ODIIID
  ODIIID
  ODIIID
  OUDDDU
  

Here is what the structure looks like (using RedPower2 bricks for the solid blocks)

Note that you need to repeat this pattern in all directions, and that the outside portions should only have a solid brick on the bottom layer. You may choose to add a torch at the intersections of the outside blocks.

You may also place a torch on the center square.

As far as movement up and down layers, I offer no useful advice. Perhaps ladders on the 'u' column blocks.