It seems to be a 'feature' of WordPress that it's designed to run on one, and only one host name.
The 'siteurl' and 'home' parameters are hardwired into the database options table, so if you hit the site on non-matching hostname you get redirected by PHP to the correct hostname. Even more irritatingly, the siteurl is prefixed to all internal links that are output to HTML. (although, perhaps that's the theme's fault - I haven't looked at that yet)
Is there a proper way to run WordPress on multiple domains without hacking the core, and without maintaining multiple options tables?