I use this script to save and restore window positions on OS X when attaching and removing an external monitor.

gistfile1.rb

#!/usr/bin/env ruby

require 'appscript'
include Appscript

DATA_FILE = File.expand_path "~/.saved_windows"

# Adium is very special.
ADIUM = "Adium"
ADIUM_CONTACTS = "Contacts"

def marshal_windows(positions)
  File.open(DATA_FILE, "w+") do |file|
    Marshal.dump(positions, file)
  end
end

def restore_process(process, name, positions)
  if process && process.exists
    for i in 0...process.windows.count
      restore_window(process.windows[i], positions[i])
    end
    
    if name == ADIUM
      restore_window(process.windows[ADIUM_CONTACTS], positions[ADIUM_CONTACTS])
    end
  end
end

def restore_window(window, position)
  window.position.set(position[:position])
  window.size.set(position[:size])
end

def restore_windows(processes)
  positions = unmarshal_windows

  positions.each do |name, positions|
    if (processes[name])
      restore_process(processes[name], name, positions)
    end
  end
end

def run
  sys = app("System Events")
  
  if ARGV.size != 1
    puts "Usage: windows (-save|-restore)"
  elsif ARGV[0].match /^-s/  
    save_windows sys.processes.get
  elsif ARGV[0].match /^-r/  
    restore_windows sys.processes
  else
    puts "Unknown argument #{ARGV[0]}"
  end
end

def save_process(process, positions)
  count = process.windows.count
  
  if count > 0
    name = process.name.get
    
    if name == ADIUM
      positions[name] = {}
      positions[name][ADIUM_CONTACTS] = save_window(process.windows[ADIUM_CONTACTS])
    else
      positions[name] = []
    end
    
    for i in 0...count
      positions[name][i] = save_window(process.windows[i])
    end
  end
end

def save_window(window)
  {:position => window.position.get, :size => window.size.get}
end

def save_windows(processes)
  positions = {}
  
  processes.each do |process|
    save_process(process, positions)
  end

  marshal_windows(positions)
end

def unmarshal_windows
  File.open(DATA_FILE, "r") do |file|
    Marshal.load(file)
  end
end

def usage
  puts "Usage: windows (-save|-restore)"
end

run