Can't remember where I found this, but it works a treat.

AppleBinaryPropertyList.rb

require 'iconv'

module AppleBinaryPropertyList
  
  MIME_TYPE = 'application/octet-stream'  # Don't know what to use, so use a very generic type for now
  
  CFData = Struct.new(:data)      # For marking strings as binary data which will be decoded as a CFData object
  
  # Convert a Ruby data structure into an OS X binary property list file. (.plist)
  # Works as you'd expect. Integers are limited to 4 bytes, even though the format implies longer values can be written.
  # Strings are assumed to be in UTF-8 format. Symbols are written as strings.
  def self.convert(data)
    write("", data)
  end
  
  # Alternative interface which writes data to the out object using <<
  def self.write(out, data)
    # Find out how many objects there are, so we know how big the references are
    count = count_objects(data)
    ref_format, ref_size = int_format_and_size(count)
    
    # Now serialize all the objects
    values = Array.new
    append_values(data, values, ref_format)
    
    # Write header, then the values, calculating offsets as they're written
    out << 'bplist00'
    offset = 8
    offsets = Array.new
    values.each do |v|
      offsets << offset
      out << v
      offset += v.length
    end

    # How big should the offset ints be?
    # Decoder gets upset if the size can't fit the entire file, even if it's not strictly needed, so add the length of the last value.
    offset_format, offset_size = int_format_and_size(offsets.last + values.last.length)
    
    # Write the offsets
    out << offsets.pack(offset_format)
    
    # Write trailer
    out << [0,0,offset_size,ref_size, 0,values.length, 0,0, 0,offset].pack("NnCCNNNNNN")
    
    out
  end
  
private
  
  def self.count_objects(data)
    case data
    when Array
      data.inject(1) { |sum,x| sum + count_objects(x) }
    when Hash
      # Note: Assumes that the keys aren't a Hash or Array
      data.length + count_objects(data.values)
    else
      1
    end
  end
  
  def self.append_values(data, values, ref_format)
    case data

    # Constant values
    when nil
      # values << "\x00"
      raise "Can't store a nil in a binary plist. While the format supports it, decoders don't like it."
    when false
      values << "\x08"
    when true
      values << "\x09"

    when Integer
      raise "Integer out of range to write in binary plist" if data < -2147483648 || data > 0x7FFFFFFF
      values << packed_int(data)

    when Float
      values << "\x23#{[data].pack("d").reverse}"

    when Symbol
      append_values(data.to_s, values, ref_format)

    when String
      if data =~ /[\x80-\xff]/
        # Has high bits set, so is UTF-8 and must be reencoded for the plist file
        c = Iconv.iconv(PLIST_TEXT_ENCODING, INPUT_TEXT_ENCODING, data).join
        values << "#{objhdr_with_length(0x60, c.length / 2)}#{c}"
      else
        # Just ASCII
        o = objhdr_with_length(0x50, data.length)
        o << data
        values << o
      end

    when AppleBinaryPropertyList::CFData
      o = objhdr_with_length(0x40, data.data.length)
      o << data.data
      values << o

    when Time
      v = data.getutc.to_f - DATE_EPOCH_OFFSET_APPLE_UNIX
      values << "\x33#{[v].pack("d").reverse}"

    when Hash
      o = objhdr_with_length(0xd0, data.length)
      values << o # now, so we get the refs of other objects right
      ks = Array.new
      vs = Array.new
      data.each do |k,v|
        ks << values.length
        append_values(k, values, ref_format)
        vs << values.length
        append_values(v, values, ref_format)
      end
      o << ks.pack(ref_format)
      o << vs.pack(ref_format)

    when Array
      o = objhdr_with_length(0xa0, data.length)
      values << o # now, so we get the refs of other objects right
      refs = Array.new
      data.each do |e|
        refs << values.length # index in array of object we're about to write
        append_values(e, values, ref_format)
      end
      o << refs.pack(ref_format)

    else
      raise "Couldn't serialise value of class #{data.class.name}"
    end
  end
  
  def self.int_format_and_size(i)
    if i > 0xffff
      ['N*',4]
    elsif i > 0xff
      ['n*',2]
    else
      ['C*',1]
    end
  end

  def self.packed_int(data)
    if data < 0
      # Need to use 64 bits for negative numbers.
      [0x13,0xffffffff,data].pack("CNN")
    elsif data > 0xffff
      [0x12,data].pack("CN")
    elsif data > 0xff
      [0x11,data].pack("Cn")
    else
      [0x10,data].pack("CC")
    end
  end
    
  def self.objhdr_with_length(id, length)
    if length < 0xf
      (id + length).chr
    else
      (id + 0xf).chr + packed_int(length)
    end
  end
  
  # Difference between Apple and UNIX timestamps
  DATE_EPOCH_OFFSET_APPLE_UNIX = 978307200

  # Text encoding
  INPUT_TEXT_ENCODING = 'UTF-8'
  PLIST_TEXT_ENCODING = 'UTF-16BE'

end