It seems that every since yesterday all unknown DNS queries have been resolving to an address belonging to Verisign... They are in charge of maintaining the DNS system (for .COM and .NET domains) so it was simple for them to add a wildcard 'A' record which redirects ANY unknown address to their site.
So basically there is no such thing as a non-resolved domain address anymore. Any DNS lookup error just means another pageview for their advertisements, and if that breaks the address resolution of a gazillion FTP/NNTP/SMTP servers, spam blockers, etc then so be it - after all if it gets them more traffic, then who cares about the side effects right?
Or maybe am I over-reacting?