I've uploaded <data> support to plist.rb. Special thanks goes to Mat Schaffer, who did the hard work of finding a test case. <data> elements are returned as Tempfiles, and can be replaced with an IO or StringIO:

# reading <data> elements
data = Plist::parse_xml("example.plist");
data['image'].read

# changing <data> elements
bin = File.open("example.jpg"){ |f| f.read }
data['image'] = StringIO.new( bin )
data.save_plist("example_data-v2.plist")

The easiest way to install is to "gem install plist"