I have observed that there are problems with all approaches that use the existence of an IP address to verify that the name being looked up actually exists.
If you're using opendns, then a request to a non-existent server returns an IP address anyway, the address being one for an opendns server. This is process is presumably in place so that errant URLs in browser requests take you to a "legitimate" page, i.e, the openDNS website where they can notify you of a problem.
Unfortunately, that mechanism seems to occur for ANY non-existent hostname. Appending a single period to the hostname does not seem to help.
