title: Unarchiving iOS4 data on iOS5 doesn't work
body: <p>I've archived an <code>NSDictionary</code> on iOS 4 using an <code>NSKeyedArchiver</code>, but unarchiving on iOS 5 using</p>
  
  <pre><code>[NSKeyedUnarchiver unarchiveObjectWithData: data]
  </code></pre>
  
  <p>throws this exception</p>
  
  <pre><code>*** Terminating app due to uncaught exception 'NSInvalidArgumentException',
  reason: '*** -[NSKeyedUnarchiver initForReadingWithData:]:
  incomprehensible archive (0x62, 0x70, 0x6c, 0x69, 0x73, 0x74, 0x30, 0x30)'
  </code></pre>
  
  <p>Note that unarchiving on iOS 4 works fine.  <code>plutil -lint</code> reports that the archive is okay.  Here it is, for reference:</p>
  
  <p><a href="http://www.complang.tuwien.ac.at/schani/monkeyfull.ltg" rel="nofollow">http://www.complang.tuwien.ac.at/schani/monkeyfull.ltg</a></p>