Ascii2, as the other forum members have noted, Inno Setup provides the ability to encrypt installers using the optional encryption module found here:http://www.innosetup.../isdl.php#crypt
In order to decrypt the installer (necessary for extraction), the proper password must be specified. By default, as you've obviously noticed, innounp can't support encrypted installers.
However, the post that you linked to looks like it may provide a way to work around it. I'd be interested in investigating it for the next version of UniExtract. I recommend adding your suggestion to the feature request thread to make sure I don't forget about it.