com.dropbox.preferences.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>Label</key>
	<string>com.dropbox.preferences</string>
	<key>OnDemand</key>
	<true/>
	<key>ProgramArguments</key>
	<array>
		<string>/usr/bin/ruby</string>
		<string>/Users/[USERNAME]/Dropbox/Preferences/Scripts/copy_plists.rb</string>
	</array>
	<key>RunAtLoad</key>
	<true/>
	<key>WatchPaths</key>
	<array>
		<string>/Users/[USERNAME]/Library/Preferences</string>
		<string>/Users/[USERNAME]/Dropbox/Preferences</string>
	</array>
</dict>
</plist>

copy_plists.rb

#!/usr/bin/ruby
require 'fileutils'
require 'pathname'

# Configuration
preference_files = %w(
  com.apple.Terminal.plist
  com.macromates.textmate.plist
  com.panic.Transmit3.plist
)

# Set paths for preferences and dropbox mirror
preference_root = Pathname.new(ENV['HOME']).realpath.join('Library', 'Preferences')
dropbox_root = Pathname.new(ENV['HOME']).realpath.join('Dropbox', 'Preferences')

# Overwrite newer files from preferences to dropbox and vise-versa
preference_files.each do |filename|
  pfile = preference_root + filename
  dfile = dropbox_root + filename
  
  FileUtils.cp(pfile, dfile, :preserve => true) if pfile.exist? and !dfile.exist?
  FileUtils.cp(dfile, pfile, :preserve => true) if dfile.exist? and !pfile.exist?
  
  if pfile.mtime > dfile.mtime
    puts "Copied #{pfile} (#{pfile.mtime}) to #{dfile} (#{dfile.mtime})"
    FileUtils.cp(pfile, dfile, :preserve => true)
  elsif dfile.mtime > pfile.mtime
    puts "Copied #{dfile} (#{dfile.mtime}) to #{pfile} (#{pfile.mtime})"
    FileUtils.cp(dfile, pfile, :preserve => true)
  end
end

# Delay next operation
sleep 30